Hi!
Not sure of any web tutorials available for this, but it's pretty simple, (i'm using premiere pro, not sure which version you have, but it'll be similar anyway)
all you do is:
1: drag clip/s to timeline
2: in the 'effects' panel, go to:
>video effects>Keying
here there are several options that will work, depending how good your screen is, you can choose either the "green screen key" or the "chroma key"
drag this effect down onto your clip/s
"chroma key" is pretty flexible, it allows you to select any specified colour to be cut out -> to alter this, go to the "effects controls" panel in your monitor and select whichever colour (ie: anyspecific colour)
you can play around with the options here to adjust things like: blend, colour similarity, threshold etc, to your taste
after this, just over-lay your images,

hope that helped,
Kitten ^_-