画像がパス名で画面に表示される

前提・実現したいこと

ウェブ上に画像、文字、テキストボックス、完了ボタンを表示させたく思っています。文字、テキストボックス、完了ボタンを実装してくれるAPIを使っているのですが、preambleというパラメータが文字列しかとらないらしく、画像ではなくパス名だけ表示されてしまいます。
preamble: jsPsych.timelineVariable('stimulus')を
preamble: "<img src=" + jsPsych.timelineVariable('stimulus') + ">"
にするとなぜか画像パスが'function()'に変換されてしまい、Failed to load resource: net::ERR_FILE_NOT_FOUNDになります。
jsPsych.timelineVariable('stimulus')は"パス名"だと思っていただければ良いです。パス名は合っています。

どなたかプラグインとの整合性を見ていただけないでしょうか？

該当のソースコード

javascript
1var survey_trial = {
2  type: 'survey-text',
3  questions: [{prompt: jsPsych.timelineVariable('instruction')}],
4  preamble: jsPsych.timelineVariable('stimulus')//ここに"パス名"が入ります。
5};

survey_trial plugin
https://www.jspsych.org/plugins/jspsych-survey-text/

javascript
1jsPsych.plugins['survey-text'] = (function() {
2
3  var plugin = {};
4
5  plugin.info = {
6    name: 'survey-text',
7    description: '',
8    parameters: {
9      questions: {
10        type: jsPsych.plugins.parameterType.COMPLEX,
11        array: true,
12        pretty_name: 'Questions',
13        default: undefined,
14        nested: {
15          prompt: {
16            type: jsPsych.plugins.parameterType.STRING,
17            pretty_name: 'Prompt',
18            default: undefined,
19            description: 'Prompt for the subject to response'
20          },
21          placeholder: {
22            type: jsPsych.plugins.parameterType.STRING,
23            pretty_name: 'Value',
24            default: "",
25            description: 'Placeholder text in the textfield.'
26          },
27          rows: {
28            type: jsPsych.plugins.parameterType.INT,
29            pretty_name: 'Rows',
30            default: 1,
31            description: 'The number of rows for the response text box.'
32          },
33          columns: {
34            type: jsPsych.plugins.parameterType.INT,
35            pretty_name: 'Columns',
36            default: 40,
37            description: 'The number of columns for the response text box.'
38          },
39          required: {
40            type: jsPsych.plugins.parameterType.BOOL,
41            pretty_name: 'Required',
42            default: false,
43            description: 'Require a response'
44          },
45          name: {
46            type: jsPsych.plugins.parameterType.STRING,
47            pretty_name: 'Question Name',
48            default: '',
49            description: 'Controls the name of data values associated with this question'
50          }
51        }
52      },
53      preamble: {
54        type: jsPsych.plugins.parameterType.STRING,
55        pretty_name: 'Preamble',
56        default: null,
57        description: 'HTML formatted string to display at the top of the page above all the questions.'
58      },
59      button_label: {
60        type: jsPsych.plugins.parameterType.STRING,
61        pretty_name: 'Button label',
62        default:  'Continue',
63        description: 'The text that appears on the button to finish the trial.'
64      }
65    }
66  }
67
68  plugin.trial = function(display_element, trial) {
69
70    for (var i = 0; i < trial.questions.length; i++) {
71      if (typeof trial.questions[i].rows == 'undefined') {
72        trial.questions[i].rows = 1;
73      }
74    }
75    for (var i = 0; i < trial.questions.length; i++) {
76      if (typeof trial.questions[i].columns == 'undefined') {
77        trial.questions[i].columns = 40;
78      }
79    }
80    for (var i = 0; i < trial.questions.length; i++) {
81      if (typeof trial.questions[i].value == 'undefined') {
82        trial.questions[i].value = "";
83      }
84    }
85
86    var html = '';
87    // show preamble text
88    if(trial.preamble !== null){
89      html += '<div id="jspsych-survey-text-preamble" class="jspsych-survey-text-preamble">'+ trial.preamble +'</div>';
90    }
91    // start form
92    html += '<form id="jspsych-survey-text-form">'
93
94    // generate question order
95    var question_order = [];
96    for(var i=0; i<trial.questions.length; i++){
97      question_order.push(i);
98    }
99    if(trial.randomize_question_order){
100      question_order = jsPsych.randomization.shuffle(question_order);
101    }
102
103    // add questions
104    for (var i = 0; i < trial.questions.length; i++) {
105      var question = trial.questions[question_order[i]];
106      var question_index = question_order[i];
107      html += '<div id="jspsych-survey-text-'+question_index+'" class="jspsych-survey-text-question" style="margin: 2em 0em;">';
108      html += '<p class="jspsych-survey-text">' + question.prompt + '</p>';
109      var autofocus = i == 0 ? "autofocus" : "";
110      var req = question.required ? "required" : "";
111      if(question.rows == 1){
112        html += '<input type="text" id="input-'+question_index+'"  name="#jspsych-survey-text-response-' + question_index + '" data-name="'+question.name+'" size="'+question.columns+'" '+autofocus+' '+req+' placeholder="'+question.placeholder+'"></input>';
113      } else {
114        html += '<textarea id="input-'+question_index+'" name="#jspsych-survey-text-response-' + question_index + '" data-name="'+question.name+'" cols="' + question.columns + '" rows="' + question.rows + '" '+autofocus+' '+req+' placeholder="'+question.placeholder+'"></textarea>';
115      }
116      html += '</div>';
117    }
118
119    // add submit button
120    html += '<input type="submit" id="jspsych-survey-text-next" class="jspsych-btn jspsych-survey-text" value="'+trial.button_label+'"></input>';
121
122    html += '</form>'
123    display_element.innerHTML = html;
124
125    // backup in case autofocus doesn't work
126    display_element.querySelector('#input-'+question_order[0]).focus();
127
128    display_element.querySelector('#jspsych-survey-text-form').addEventListener('submit', function(e) {
129      e.preventDefault();
130      // measure response time
131      var endTime = performance.now();
132      var response_time = endTime - startTime;
133
134      // create object to hold responses
135      var question_data = {};
136      
137      for(var index=0; index < trial.questions.length; index++){
138        var id = "Q" + index;
139        var q_element = document.querySelector('#jspsych-survey-text-'+index).querySelector('textarea, input'); 
140        var val = q_element.value;
141        var name = q_element.attributes['data-name'].value;
142        if(name == ''){
143          name = id;
144        }        
145        var obje = {};
146        obje[name] = val;
147        Object.assign(question_data, obje);
148      }
149      // save data
150      var trialdata = {
151        "rt": response_time,
152        "responses": JSON.stringify(question_data)
153      };
154
155      display_element.innerHTML = '';
156
157      // next trial
158      jsPsych.finishTrial(trialdata);
159    });
160
161    var startTime = performance.now();
162  };
163
164  return plugin;
165})();
166