Tom
Have you tried PhotoBox - you can automate the sales process, and if you then make the donation to the charity (and are a taxpayer) then you can get the extra tax added on to the donation.
Using Photobox means you can set the price you like, and the buyer gets a finished print at their chosen size, posted direct to them.
Just a thought.
Example of a pro gallery on
Photobox not mine, BTW.