There are a few things to check about your file if you are having problems uploading it:

1. It should be a valid JPEG file
2. It should be less than 8 MB in size
3. It should be in RGB, not CMYK
4. It should be under 3072 pixels wide on any side
5. It should be an 8 bit not 24 bit file
6. There should not be any strange characters in it's filename, such as " ~ ' / \ ; etc.