Paint.net
&
gimp shop

Using the programs above…
you can design the square covers, for example, of a software
box. Then make a rectangle spine for the box. Add graphics and
text, then put it all together using Scott’s Box Shot Maker.

Scott’s Box Shot Maker is also free. If you click on the view
samples within the software you can see some good examples. You can
copy the dimensions from these to get the idea of how to size
your graphics.

Michael