Hi,
My 2 cents here ...
[1] You open a gif image
[2] Pick the "Dropper" tool
[3] Right-Click the image in the color you whant it to disappear (this will select it as background color on PSP)
[4] Then Menu->Colors->Set Palette Transparency (or Ctrl+Shift+V)
[5] On the Window: "Set the transparency value to the current background color" (that's the one you just selected before with the dropper) - you can click on "Proof" to preview the vanishment
[6] Click OK ...
[7] Then Menu->Colors->View Palette Transparency (or Shift+V) to see the change ... voilá ... just save the gif file and you're of to put it for example on a web page ...
Is this what you were searching?