TWO REASONS to charge a fee when a photo is requested. 1) Organizations can charge a nominal REPRODUCTION FEE for the time and resources it has taken to digitize, make available, then time and resources to make a good photo copy or send a low res. version over email. Communication to the requestor that this does not give them rights to reproduce it is important. [small funds to help continue digitizing photos]
2) Organizations that have an image where the requester is going to publish it, reuse it, sell it on postcards, etc. and they own copyright OR even if it is an older out of copyright image can still charge a USAGE FEE. Higher amount if it is for commercial purposes or sales purposes. Lower amount or no fee at all if for a non-profit – you choose. Communication to a requester about one time usage for a usage fee is also important. An example is: a stock photo company getting a high resolution file of one of your photos and selling it repeatedly where they get the profits; or a non-profit gardening group getting a good image of one of your wonderful botanical prints and selling it on tote bags, mugs, and cookbooks for 20+ years.
I think we have a responsibility to educate users/requesters about usage rights and copyrights.