Okay, first, you make the background completely transparent. To do this, just use the magic wand tool, and drag all the white away from the image. Next, you paste on the image you want as a layer. Now press Ctrl + l to open the Layer tab. Click on the layer to make it the active one. Now, at the top you'll see a thing that says Opacity. Change the numbers to make it more or less transparent.ßubba said:Opacity.
And I'm so sorry bout double posting, please don't report me.
Now, the only problem is, you can easily save it as XCF to view for yourself, but I don't know of any image hoster that will upload it. The only one I know is http://www.upload2.com/ will host PSD files (which can handle opacity), but I think you have to download the file to view it, and then the viewer has to own GIMP, Photoshop, or another program that can open those files.