[palosanto]

Use a vector graphics application like Inkscape (free), Xara (my favorite), Illustrator. There are many free programs.  Save as svg or png if you want transparent background.
You can use photoshop but for crisp outlines better use a vector app.
An output size of around 50x50 is adequate for most uses and can be resized gradually without big jumps.  You need to do some experimentation to come up with a graphic size that suits your needs.
Don't use jpg unless you want solid backgroud and if you do make sure that the stamp color is set to null or you will get a colored square.

I'm not sure but I don't think custom fonts are supported.