I do noy understand the jpeg,Targa,Bpn,ppm format.
what do I use as the url for boince in that spot?

I don't quite understand what 'Preference Spot' you are referring to.
I assume you are referring to the 'Your Account' page, but there are many preferences there, which?

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but there are many preferences there, which?

Probably "URL of background image" and "URL of logo image" in SETI@home Preferences.

Just put up a picture (in a format supported such as BMP - like Windows wallpaper) on some website (you can get a free website at www.geocities.com or other places) and put the URL to the picture in the "URL of background image" field.

For example, if you create a site at GeoCities with a username "diane", your website adress is "http://www.geocities.com/diane". If you uploaded a picture called "test.bmp" then just enter "http://www.geocities.com/diane/test.bmp" in the field.

The next time you do a update on SETI@home, "test.bmp" will get downloaded and you will see your picture used as background when you view graphics/screen saver.

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