Does it really make a difference when texturing if you use jpg or a bigger file format?
Any issues with image/compression quality will show on the model just as in the 2D source. I wouldn’t recommend using JPGs generally, but they wouldn’t a problem if high quality and/or not viewed too closely in the final image. But in the end, if your satisfied with the result, use whatever you prefer.
Depends on what you use the texture for - for close ups it’s always nice to use huge images in a maybe losless format, for unimportant parts of the scene or objects far away .jps might suffice. I personally like to use .pngs for the support of alphachannels and as long as I don’t run into memory problems I stick with them. But in most cases, .jpgs at a low compression level (that means, high image quality settings) should be ok.
While there is no discernible difference between an “original” and a first generation very high quality jpeg image, you should probably move to .PNG. Why? Let’s say you need to make a tweak to that texture. Open, paint/edit, and save. Over the course of your scene/production creation you have to do that a few more times. Each time, you lose a little information and the quality drops, even if you’re using best quality. Rather use a non-lossy format like .PNG (or .TIF).
Hard Drive space is very cheap. Use it.