Got some static images (jpeg + tiff) that I need to float over a video clip, playing in the background?

I can do the transparent white or black (depending upon the static image background) but it also takes out some of the objects' white and black (obviously!)

Though I could do it using an alpha channel - but I cant seem to?..

Any chance of a step by step guide on how to take an object in an image (jpeg or tiff or psd) and make the background (whatever colour) transparent - without affecting the object? It?s late, and I think I'm missing something somewhere?. (probably brain cells?.)

Thanks

S