There are a number of questions on whether or not to use blobstore, such as Store Photos in Blobstore or as Blobs in Datastore - Which is better/more efficient /cheaper?. In all of the cases that I have read, the image is something which changes relatively rarely, such as a profile image.
I need to store thumbnail images for the first frame of many different user-written programs. Every time that they change their program the thumbnail will be recalculated. As such, it could happen at a rate of up to multiple times per minute.
Blobstore's support for read/create/delete but not update suggests to me that it is not intended for this type of use case. Is it?