There are a few reasons that this may be occurring. The most common two are:
- Make sure that your image is smaller than 5MB in size
- Make sure you don't have a double extension -- for example, picture.jpg.jpg
If neither of those is the cause of the problem, make sure that you're able to upload media elsewhere on the internet; there may be a problem with your connection.
If it otherwise seems fine, please let us know, and we'll be happy to help figure out what's going on.