Hi there,
This is usually caused by saving the output HTML file in a different location, but not putting the images in the same location, so the browser can't find them. Copies of the images need to be in the same location relative to the HTML file as the original images were relative to the exercise file when you imported them. So for example, if your original images are in the same folder as your .jmt JMatch file, then the copies need to be in the same folder as the .htm output file.
Hope this helps,
Martin