Making Images Tranparent

We have a .NET application which by user action spawns two forms (681 x 281), one on top of the other. The bottom form contains a user-selected JPEG image (also 681 x 281). The top form contains an all black JPEG image (also 681 x 281) which serves as a background for programmatically integrated JPEG (74 x 91) images randomly taken from a library of about 50. These are integrated with the black background insofar as the opacity of the background and 13 images can be changed globablly with an opacity slider.

The smaller images, made with CS3, have black backgrounds so that they blend with the top form's all-black JPEG. They can be manipulated by the user, moved around, pieced together somewhat like a puzzle, and so on. Our problem is this: When they are moved around, pieced together, etc., the black backgrounds can be seen overlapping adjacent images, greatly undermining the quality of the design. We haven't been able to use the color-exclude key to make the black backgrounds transparent, and GIFs and PNGs don't work, probably because the development environment doesn't translate them the way a browswer would, for example.

Does anybody have any ideas on making the 13 individual images' backgrounds transparent?

You can download a copy of the software and see what happens for youself at http://www.aquaware.org/ALIIX/ALIIX.exe

Thanks

Peter