Limit on the number of fixed images that can be configured for a form template

There's no fixed limit in the code to prevent a certain number of images from being included, although we'd always advise testing and monitoring performance of image-rich forms.
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If an image will not load and shows instead as a broken image, then it could be due to an issue with a character in the filename. Try removing any special characters, including ampersands &, and try again.