Nice job. Focusing on your background question, to my eye, the background tone is a little dark. The tone works well for the dog in the front, but the tone seems too close to the middle dog and could possibly lighten for the dog in the back. Possibly a gradient that works from center, or working with a gradient that reinforces the composition may push the dogs further forward. The scale of the texture in the background seems a little large, scaling that down, so that you see it as a texture and not its constituent parts, might serve to simplify what you are seeing. That scale reduction may have the result of darkening the texture, but you can take it down reducing the opacity or saturation. The middle dog kind of melds with the dog on the right due to their similar colors, to me, this affects the composition, I see reflected light back to the middle dog at the leg, but could that reflection be used higher toward the head to clarify the order in space. The similar colors in that area formulates an isosceles triangle low at the center which to me, creates a bit of confusion within the composition. I see this more evidently in the thumbnail. One might also try lightening the dog to the rear further using a little license. I hope you find these comments useful. Best.

Beatiful composition and picture. Things that I would look at and fix are mainly the biggest dogs legs and body, it doesnt look very anatomically correct even if the dog is overweight. White dog looks the best in my opinion but he lacks contrast and looks like overexposed photo but this is easy to fix and propably part of your painting process. The middle-ones skull seems off to me, so id look at it to. Overall this will be a beatifull painting. Backround i would leave as is, or work on last if it feels like it.