An image might still be a valid image and still contain malicious material such as embedded comments or other (undefined) attack vectors. One -- albeit old -- example was the [GDI JPEG vulnerability][1] in Windows from 2004.
I think your primary concern is to ensure that your image-acceptance mechanism is simple and tight. You'll want to watch for file permission errors, resource exhaustion (running out of space, tying up network resource, etc.) and path attacks (eg. potentially overwriting files).
Furthermore, ensure that any string-handling checks are strictly and precisely defined. There was a [recent question on sec.SE regarding attacking ASP upload code that used a less strict string searching mechanism][2].
Finally, search this site for the term "upload". Some fine examples which can give you a clear picture (no pun intended!) of all of the challenges you may face: