Uploading New Background Images

So, firstly got to start by saying I’ve been making this site on and off for a few weeks and I absolutely love this theme. By far the best thing I could find and decently priced too 🙂

Now that I have your attention…

I can’t seem to upload a new header/footer image. After attempting to upload using the same size and format as the originals, my site is completely white (which, on a side note, seems weird as the page background is set to #000000). I remember that when it was default settings that there was a preview box for each of the original images. After uploading my own, there are no preview images but I can still see the red icon to delete the image that should be there.

Screenshot of error: http://i.snag.gy/vlJIv.jpg

Any idea what I’ve done wrong? More than happy to create an admin account for a forum support admin to take a look.

Many thanks!