I don't advise you to use paint but well , Good job as a first siggy.
If you want a good backgrund and matches to the render , You can follow these steps "Only in photoshop".
1- Copy & paste your render like 3 times every1 in new layer.
2- Put them behind each other and resize them to bigger untill you remove the white/<color> of the bg.
3- Click on CTRL and check them all & press on right click & click on "Convert as 1 picture" i guess it named like that.
4- Use smudge tool & Smudge the picture until it look good.
I'll do tutorial when i have time.