Ah, the last ones are sort of puns X)
Just some tip (I personally use them, so I'm just spilling the beans..)
Pick a right render. I'm not saying your sork is nuts, but your choise with renders suck coz they are badly cur, or just not up to the mark. If you want some fine renders, go to this site-
https://s36.photobucket.com/albums/e24/svial/Anime%20render/?start=160
This must have all the renders. If not, searvh the following in the search bar : (Name of render goes here) render image.
If you can't find a render, but found an image with plain background such as this http://dragonballzgamesonline.net/wp-content/uploads/2010/06/Goku-Hame-Kame-Picture.jpg ,there is a cheap trick to deal with it.. Outcome won't be that nice, and you need to be careful that there is not much color on the image you want to be rendered..
• Open the image
• On the layers dialog box, you'll find the "Background" layer. Double click it.
• Another box will appear, and the name will be "Layer 0" by default. Click OK. Now the layer lock will be removed.
• Select the "Select by color" tool, and click on the white part of the image you want to remove. You will notice some black lines moving around the border.
• Go to Edit > Clear
• now you can save it as a png image.
Note : Real rendering is not done like this. You use the pen tool for that.
Note 2 : If your render is still rash, try blurring it, or outer-glow.
Now that I've covered the renders part, to the background-
Like DN said, use the "Clouds" effect. Other than that, you can also use brushes, solid BGs, gradients etc.
You can also use un-rastirized layers for your advantage. Also, smudge tool is your friend. After you have used your brush, try the various options in the filters drop-down list
Hope you read this all and learn something