This is the simplest tip I've ever given. But, it could make a huge difference on how your Profile Page Cover image will look. If you're interested in adding a nice cover image for your Profile Page, the best image dimensions are:
945px in width by 245px in height
Otherwise, the clarity of your image will be blurred or the wrong proportions. If you go to my profile page, you'll see an image that is 100% crisp. You will see some loss of clarity below 768 in width.
Additionally, try to leave a little white space on each side, to accommodate better viewing on mobile devices.
These are not the dimensions suggested on Ning's 3.0 Help pages, as we should be able to use 880px in width. Some images that are real photos will look okay at 880px. You'll notice that Ning's default cover image has lost a tiny bit of it's clarity.