@Alexfili - The paintings are backed by glowstone
and in context they're a lot brighter than the rest of the room. It isn't possible without heavy modding to make them any brighter. Placing torches next to them does nothing.
But hey, the limitations of Minecraft add to the charm of the project and make it more amazing to achieve things. Try not to set your standards too high.
@Splat - I'll have to tweak the images again anyway, and yeah I agree that some graphics are a little too clear compared to the rest of the images 'cause they were done at the same resolution as Minecraft paintings.