I think it would have been easier to select the truck and replace the background completely than to try to fix all that in my opinion. Easier if you have photoshop which I assume you don't since you used a Paint program.
Don't get me wrong, I still think you did a good job. There's really not a whole lot to touch up. Mostly just the grass on the left. Awesome truck btw... and welcome to TPF!
Thing I see other than the grass/sidewalk that stands out to me is the corner of that building... It needs to be redone. You can tell a repeated pattern, as well as see that the corner has a curve where you edited it.