Neon: To make this one, I used this blank template: [link]
Open it in an art program,and start drawing whatever you want inside of it. If you want it to keep that jagged shape and not just be a square, you need a program that can handle transparency. MS Paint can not do that.
I used a free art program called GIMP 2, and I saved it as a PNG file.
To use them... I think you need a premium account. On the right column of the stamp page there is a thing that says details, and a box called "Thumb". Copy and paste that code onto a specific column of your profile page. It will show up as a stamp.