I suppose it can be offtopic, but I didn't find this info anywhere. I am going to build a GAE app that will heavily work with Picasa as an image hosting. It will add new images, fetch existing, update them, work with comments etc. I have read Picasa API docs and I think it can be integrated and used with GAE quite quickly, easily and conveniently. So, is that a right choice for a highloaded web app? What a restrictions or quotes for using it?
I just read this blog article: http://www.carlosble.com/?p=719 Long story short, the guy wasted 15 grand because he did not do his research and found out GAE was not appropriate for this needs.
It sounds like your app might hit the quotas imposed by GAE, I guess it would depend on how popular your app is. Here are the quotas http://code.google.com/appengine/docs/quotas.html
I dont know of any service that it is similar to GAE and is free to start out with. You just need to make sure this is the right technology if you plan to invest heavily in it.