You need to save the background picture, then make your picture
transparent, then copy it to the background image.

What I would do is first find a background, open it up on MS paint,
then, open a second MS paint and draw the pic on that one. Once you
have that done, just copy your pic, switch to the paint with the
background and paste it(there will be two options after you paste
it...just click between them and see what you like better).

Well couldn't you open up the program with the backround, paste your
drawing on it, and erase the white backround to show the image? That's
how I do it

You would have to copy and paste the background first. Then you would
have to draw the picture on it. There isn't really any other way to do
it.