Hi Ukyu, I assume you are using the manage attachments uploader? (click 'go advanced' and then 'manage attachments').

If your file is not uploading then it is either an unsupported file type or the dimensions are too large (I don't know the limit but it should take anything up to 3,000 by 3,000 px) or the file size is too large (again I don't know the precise number but anything up to 3 mb should be pretty safe). If you're using a supported file type and are within the required dimensions it may be a problem with the uploader and you may have to wait a little while and try again.