Can't change background of mod pack

[NunSuperior]

Technic Platform issue (web site):

I'm trying to replace the existing background for my mod pack but the web page isn't uploading the new image. When I click "browse" and select the file, then click "Save modpack" no change occurs and the original image is displayed. If I delete the original image, and save the mod, then try to upload the new image the old one re-appears. It is sized 880x520 JPG (52KB). No errors are shown.

Thanks for the help. Great service you've got here.