Do you know what the onboard graphics are? A graphics card would handle larger files more easily, and you wouldn't need to pay a lot, you can pick up a 16/32MB ATI Rage for £20 - £30, but wether you get one or not depends on how much photo editing you do.
You would have to disable the onboard graphics, which should be done easily by changing one setting in the BIOS. Then slot the new card in and connect the monitor.
Another 128 MB system RAM will probably speed your system up a bit too for about the same price. I'm not sure which (graphics or RAM) would benefit you the most though.