1. What are the point of high res. textures?Hi resolution textures are more visually pleasing because greater detail can be captured since more pixels are available.
2. How come the game can use a .jpeg file and .wal files but bsp can only use .wal?.wal are low-res textures that were the standard texture format in Q2. PB2 introduces the ability to use hi-res textures in the form of JPEG and TGA, however TGA is an uncompressed image format and takes up a lot of unnecessary space. Generally when hi-res textures are distributed, they are in the .jpg format. If you are making new textures for your map, it is best to create both a .jpg and a .wal (using wally or whatever that .wal export plugin for psp/ps/etc.). The reason to create a .wal of your hi-res jpg, is so that you can preview your brushes with a texture in BSP. I also believe that arghrad(RAD) uses the .wal for lightmaps or phone shading, someone will have to back me up on that.
3. What are TGA files and how are the used in the game and in bsp?.tgas have always been used in Q2, but only to present the skybox. PB2 allows .tgas to be used as a texture and we're able to take advantage of the alpha channel (in 32bit .tgas) to create better transparency effects.
4. In the textures folder, there are folders pertaining to the map they are used in and such. How does the map know to go to that folder and not just got to the pball texture folder by default?Most of the textures are found in the pball folder, textures found in other folders could almost be considered "third-party" textures. You can use any texture in any folder in your maps, and it is advised that if you are creating your own textures that you place them in a seperate folder so that they don't clutter the existing folders.
5. Some of the maps i have downloaded in the past are missing they textures. If there anyway I can download them?Some (not all) are available at
http://dplogin.com/files/textures/