Clarify that "Image resource" can be any supported image format [0va7u6]

[WilcoFiers]

It would be useful to put a note into the "image format" definition that explains that it is relevant to all supported image formats in the browser, such as PNG, JPG, SVG. It might also be useful to mention something about how there are differences between what image formats browsers support.

Not sure if images of text is the only rule where this is relevant. Should look at that as well.