Why can't you just "slap on your jpg"? As long as the texture is ultimately volumetric, a picture file can work just fine. (See a few of the rain tutorials to see what I mean). Otherwise, if your jpg doesn't look good as a volumetric, consider modifying it, or adjusting the base density of your volume material (which can help a lot) or playing with the fuzzy factor, or abandon the jpg and play a little more with the DTE and making your own material that way.

Could be worse, could be raining.