Nik, adding to Mike's reply, at the bottom of the upload window there is a list of allowed file types and maximum file sizes. 1200x800 is about as big as you can reliably make JPGs, and even then you might have to reduce the quality.
RIOT, for example, will work out the best quality you can use to stay within the size limit. It the resulting image quality is too poor, reduce the size and try again.