What program? In Thunderbird you can do it in a number of ways. I created a
simple html one and saved the file. Then in the Account settings (Edit | Account
Settings) on the line with your user name (just above the Server Settings) you
put a tick in the box that says Attach this signature and direct the program to
the file location. If you use graphics your html must use realtive links. If
they don't display go back to the html code and change the code. Other programs
such as Evolution work in a similar way. You can also get som signature Add-ons
for Thunderbird that change it. I think that you can even use random ones from a
text file but it has been a while since I did this.