Hi Ange,
 
This happened to me a long time ago and I finally figured out that my image was too big for the size limitation and rather than giving an error message, my image just wasn't there after 'appearing' to upload.
 
Go back and check the size, the first thing to try if you are using Photoshop is that when you save the jpg, change from a MAX of 12 to 10 in the resolution box.  That usually is all you need to get it down.  
 
Hope this helps...So check the size and be sure the file you are trying to upload is a jpg and not a different type of file.  If neither of these fix your issue then I'm afraid you'll have to wait for someone smarter than I