I have a jpg that I have "painted" the background white, eliminating clouds, sky, etc. How do I now make that transparent and with what software?

I use GIMP to make the backgrounds of my photos transparent.

You can download GIMP at http://www.gimp.org/downloads/ where you will find the source code, and a link to the installer for MS Windows.

If you are not familiar with graphic editing software, you should look at the GIMP pages for tutorials. I think you can find one that will tell you how to make the background of a graphic transparent. Just notice that the volunteers that made the tutorials may not have updated the tutorial to match the latest changes to GIMP.

If you do decide to use GIMP, you should be able to get it running, and follow the tutorials, and then ask questions here or on a graphics forum if you need additional help.

The jpg file format does not have an option for or support for transparency.

You must use the gif or png format which both support transparency.

GIF format is limited to 256 colours.
PNG supports 8 bit or 16 bit color,

Depending on the graphics editor you use and the type of image you have you will have to use on or the other, but not JPG.

Mike...