I have been using the Swiss Post Card web site for several years, and I am quite impressed with the way they have continually improved it. What started as a very simple "upload a picture and send it by mail" has developed into a very nice personal and business resource. With the latest version, you can upload images for the front of the card, the back/text area, and even for the "stamp". Cards cost 2.50 Swiss Francs each, and when you compare that to the price of a card and postage here, that's a good deal. It's a great way to send pictures to family and friends who are not computer-savvy, so we can't just email digital photos to them, but we don't use it only for them, just about everyone still enjoys getting something fun in their mail box.
The Swiss Post Card web page lets you upload pictures in a variety of formats (I generally use JPEG), and then has some simple tools for placing, sizing and stacking those pictures in the post card image. For more sophisticated photo editing, such as rotating, cropping and special effects, you have to use a program on your computer and then upload the finished image.
Creating the images for the cards has led me into learning about image manipulation programs and creating photo collages. I initially used Adobe Photo Shop Elements, which worked quite well, but since I seldom boot Windows on any of my laptops any more, I recently turned to the obvious Linux alternative - GIMP, the GNU Image Manipulation Program. A quick web search for "GIMP Collage" got a me list of pages with tutorials, tips and video demonstrations. With a little bit of reading and experimentation I was creating just what I wanted in a few minutes.
Good stuff, both of these!