Free Download with purchase!

[coolman]

I am having some difficulty getting this right!

I would like to offer a free wallpaper with the purchase of software that I sale on my site. I want the download of the wallpaper, in jpg format, to show only when the software is paid for. The download it's self should not have a price because it is free with perchase of software. Also, the big problem is that there are many diffrent wallpapers, depending on what software you buy. I do not want to use a radio button and allow the customer to choose which wallpaper they want. Summing it up, the customer should only have one wallpaper available for download depending on the software they selected.

Any help you can give me will be greatly appreciated,
Thank you.