I have made a few cards recently using this tutorial for making your own digital stamps. I realised though that I haven't been doing it exactly as that says because it's not very clear, and not so easy to follow, so I am going to share with you the way I do it. My instructions are based on paint shop pro, but I think they should be similar for other photo editing programmes.
Select your image. Crop and re-size it to how you want it.
Adjust the brightness and contrast, making the contrast quite high.
Adjust the threshold so that you are left with a black and white image. Threshold can be found under 'Adjust' and then under 'Brightness and Contrast'
Create a new image, making sure it is the same size as your image. In PSP when you create a new image it gives you the option of what colour you want to make it. Here is where you create your background. I just play around until I find something that I like.
Copy and paste your image over the top of the background. Then use the 'magic wand' to select the white parts and delete them.
Now you can add whatever extras you want to such as a border or text.
This image is of Billie Joe Armstrong from Green Day, taken by me when I saw them last October.