It's hard to say, I'm afraid! Does it work if you upload the image to somewhere like Imgur? If it does, can you send me the link so I can investigate what's going wrong? 
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Teapot
Teapot
A couple of things to consider: Make sure the image isn't too large – over about 5MB, as the server will reject it for being too big. Also make sure you haven't accidentally renamed another format of file – if you have a PNG but it ends in '.gif', then the site will be confused and reject it.