Doc,
Without trying to sound too smug, putting this stuff together really is child's play. The hardest part is knowing enough about Photoshop Elements to be able to cut and paste images onto one another and to alter their sizes. I've been using Photoshop for some time, and if you want to get into making this stuff, then you may need to get a tutorial. The real beauty is, is that once you've mastered it, you can really have some fun. The stuff shown above literally took me a nights work.
To start with I used Niub Niub's decal sheets as a template, everything I do conformes to his sizes, that way I know it will all be to scale. I use one template and simple pate my images onto that template size.
Materials wise all you need is a cutting board, some mount board (thick card), a good craft knife, a pair of scissors, a pack of "one sticker per page" A4 sticky labels and enough ink in your printer (ink represents the highest production cost, so make each sticker count).
When I am looking for images I use Google Image Finder. If you go to Google
http://www.google.co.uk/ig?hl=en on the top left corner you'll see some tabs (WEB, IMAGES, VIDEO, MAPS, NEWS etc, etc) select image, then enter your search requirements. A page of images will pop up, and you have the oiption to see a selected image in full size, just save the images by right mouse click (Save Image As) to your desktop or on your hard drive and there you have it your own image library.
It helps if you type in the right search requirements. I usually enter in specifics like "Imperial Cruiser, or Death Star, alternatively I enter in variables like "Scifi wall texture" or "Hangar bay interior", you'll be surprised what comes up.
If you want my decals here they are...
http://www.flickr.com/photos/30029551@N06/sets/72157619465883345/I will add more as I make them.
Also here is a link to an earlier tutorial I put together for my Star Base project. I used a similar tecnique, except with Mount Board it's much simpler.
http://www.flickr.com/photos/30029551@N06/sets/72157614287992670/Have Fun!!!