One of Google App Engine's features includes dynamic image resizing. You can store an image and simply modify the URL to request resized or cropped versions of the photo. On App Engine this can be done efficiently.
From the image transformation docs (scroll down on that page to see it), the arguments for resizing and cropping:
The available arguments are: =sxx where xx is an integer from 0–1600 representing the length, in pixels, of the image's longest side. For example, adding =s32 resizes the image so its longest dimension is 32 pixels. =sxx-c where xx is an integer from 0–1600 representing the cropped image size in pixels, and -c tells the system to crop the image.
This same technique and syntax appear to apply to any photos you have on Google+. Hover over an photo from one of your albums and copy or open the image URL in a new tab. Look for the /sXXX/ portion of the URL.
Resized to be 100px, cropped square (s100-c)
This makes it easy to resize an image for posting or using elsewhere.