The RAF Museum in Hendon has a huge collection of paintings that can be loaned out to stations. I believe the only cost is that for transportation and insurance of the art. You used to be able to browse the art archive online but I can't seem to find the link anymore.
Alternatively, the MOD art archives offer art to MOD establishments on similar terms, however CDS/CJO generally plunder the collection for their own premises. Loads of portraits of dead admirals available apparently.