There's a freeware Windows utility called
Replace that can do what you want.
The site has good info on how to use it...but here's the magic part:
Replace
(tr class=g2)(td class=s4)(img src="
With
(tr class=g2)(td class=s4)(img src="photos\
(you'll need to replace the ( and ) in the example with < and >. If I include them in the example, the board takes it as HTML coding and doesn't display them)
...replacing "photos" with the name of your photos directory.
If you want to get really funny, you can also replace .bmp with .jpg, convert all your photos to .jpg, and save a bit of space on the server.
Hope that helps!
--Eriq.