I have noticed that if the jpg, png, etc file had been edited in a program that allows layering and guides, e.g. Photoshop, that unless the file is flattened, and the guides removed before saving the file as a jpg or png, etc, it will not upload to Roll20. jpg files are always flattened before saving, but it is not clear to me if there is any leftover information in the file header that is confusing the Roll20 uploader. As for guides, these are saved inside a jpg, and even though they do not appear in an image viewer, they will reappear inside Photoshop if the jpg is opened. I have encountered many jpgs on the internet that when opened in Photoshop, an image guide or two appears. Try opening your file inside a program that will allow you flatten it and remove all guides.