As far as strictly modifying an existing image, ChatGPT is probably the best. Most of the others will let you upload images and you can get a rough description to prompt with but you'll get nothing but nonsense maps back from it.Which web-based service let's me upload an image and modify it in some way?
I mainly want to use this for maps. For instance, upload a jpg that shows a map of a mountain, then generate a decorative border and cartouches in the white space around it