The Roll20 text editor has issues with images copied and pasted from a local source (i.e. your desktop). They may initially appear, but then disappear when the text is saved. In general, images copied from sources that are online (ex. Right click an online image, copy image and paste into text editor without ever downloading) do not seem to have this trouble. Game images that are stored in a forum thread or your user library are even more dependable. My assumption has always been that some vital metadata is missing from a desktop image vs an online source.