[mod_python] Session class or Memcache to reduce load?

Hello, I am sorry if my question is too basic. I would like to reduce the
load on apache 2.0 (on 2.6.9 linux) that is running with prefork MPM.
There are two main things that are causing the load:

a) Thumbnail images that are requested repeatedly 
b) A simple DB query is necessary to locate an image file after the request.
The result of the query does not change. DB is located on another machine.

My first question, do we need to cache \thumbnail images at all, or the
file-caching by the OS is sufficient?

Second question, to cache the results of the query, should we use
mod_python's Session class ( wich will use DbmSession since we are using
prefork MPM), or memcache?

For certain reasons in the application logic we cannot use apache's
mod_cache.

Thanks

Alec.