backgrounds on png pix

I am coding a tabbed window using java/swing

 Why is it that some png pictures somehow blend their backgrounds with
their
surroundings' and some other don't?

 Take a look at both examples

 http://www.geocities.com/tekmonk2005/pix/example00_OK.jpg

 http://www.geocities.com/tekmonk2005/pix/example02_BackGroundNotGood.jpg

 When users click on "Save" the entire background gets higlighted
(including
the png pix' itslef), but when they click on "Q & Ans" the background of
the picture itself doesn't.

 How can you make it consistent like "Save"?

 If you have, say, a jpeg pix and you converted to png using gimp. What
property of "png" graphix should I look into as I ex****t the file?

 Thanks
 lbrtchx