I don't know paint.net personally, but I think it uses some kind of special file type that only itself can read. This is to save some features only the program itself can use. Try firs writing .png after the name of the file. Like: example.png
Then try again. If it still does not work then try finding something that I think is called a scrollbar. It should be right under the place where you type the name of the picture, and should contain something like "Paintnet picture (.pnt)"
Also what does it say when it refuses to upload the file? "Could not connect to server" or "Could not read file, please save in JPEG, JPG, GIF or PNG"? |