If you wanna make textures for HL2 by photographing, would it make sense to shoot a picture in raw instead of jpeg or is it just a stupid idea, because jpeg is perfectly good in this case.
take the picture in as high quality as you can, so you have as much data as possible to work with for photomanipulation. JPEG isn’t optimal for this because of compression. When you’re done making the texture, use whatever format is highest quality within the file size requirements of the Source engine. It’s better to downscale it after you’re done working with it than before you start.
Well, if you’ve got a DSLR or Digital Camera, then RAW is optimal for editing, and cleaning up the image. Also good for adjusting tones, contrast and levels in lightroom, photoshop camera raw, or something! 
If you can’t shoot RAW, then JPEG will be fine for most textures as long as you don’t compress it.
The only advantage for RAW over JPEG in this case is pre-editing stuff!!
Unedited photos are useless as texture maps 99% of the time. The most basic reason being the lighting in the photo isn’t going to match the lighting in game.