Options for Image Caching
Blobstore is fine.
Just make sure you set the HTTP cache headers in your url handler.  This allows your files to be either cached by the browser (in which case you pay nothing) or App Engine's Edge Cache, where you'll pay for bandwidth but not blobstore accesses.
Be very careful with edge caching though.  If you set an overly long expiry, users will never see an updated version.  Often the solution to this is to change the url when you change the version.
I am running a website on google app engine written in python with jinja2.  I have gotten memcached to work for most of my content from the database and I am fuzzy on how I can increase the efficiency of images served from the blobstore.  I don't think it will be much different on GAE than any other framework but I wanted to mention it just in case.
Anyway are there any recommended methods for caching images or preventing them from eating up my read and write quotas?