Start off my getting this image.

Select the pologanal lasso tool.

Start by clicking and selection the line around the image you want rendered.

This is what it should look like.

Then right click on the image and goto "select inverse".

Then press ctrl+x or goto edit-> cut.

You shoudl still have some little holes left make sure to do the same thing to those holes as the background of the image.

This is what it should look like.

You can also goto Layer->Matting->Defringe and set it to 1 or 2 pixles to make then render more precise. Which I did not do in this tutorial.
I am making a applying a render to an image tut. soon so wait and it will be up.
-KillJoY_gs