I'm not particularly good with image manipulation myself, I just learn as I go, but a great tool to use is GIMP (GNU Image Manipulation Program:
[You must be registered and logged in to see this link.] It's basically an open source version of Photoshop and allows you to do things like transparency, layers, stretching/warping etc.
My B7 covers were basically publicity stills I found on the internet that I modified by adding the program and laserdisc logos. I did a little bit of image manipulation to add some fades too.
For the Memento one I did the same, though I did a little more image manipulation involving rotation, blurring out existing text and moving the logo. Once I had the image I found a picture of a black, gatefold laserdisc and pasted my cover over it and warping/squeezing the image until it looked like it was part of the laserdisc.
I think this is how the Lost in Translation one was done too, but I think the Wrestle Mania mock up was something much more real...either that or he's a real wizard at photoshop : )
Check out GIMP, it's can be a bit confusing but there are a lot of good tutorials out there. Like I said, I'm no expert but after experimenting with it for a short amount of time I found I was able to produce decent results.