Macros are also those pictures you see popping up in forums that say things like "0WN3D" and other stupid things made to make fun of people or be otherwise amusing. Typically this will be a picture gleaned from wherever, that depicts a weird person or someone making a stupid face. Text is then doctored into the picture, which will make it funny. For example, there is a picture of a nerd sitting at an old Apple computer, and the text reads, "U SUX @ FIREWALLZ CUZ I GOT r00t, LOLERS F4GG0T."
We all know that Macro means large, but it can also be interpreted to mean visible. Macros typically scream visibility, with attention-catching pictures (a kitten in a beer mug, Arnold Schwarzenegger, goatse.cx, etc), and large, bolded text with a bright border to make it stand out even more. Many a flamewar has turned into a war of macros, and many many seconds have been lost to long pageload-times.
Macros are also called macros because they are typically pre-made responses-- shortcuts, if you will, to a certain kind of situation, and the reason they catch on is because there are so many lamers out there.
Many argue that the macro was born in the confines of the infamous SomethingAwful Forums. This is possible and plausible, as many many memes have been born there-- Admiral Ackbar (of It's a Trap! fame), spoofs on Cthulhu, The Terrible Secret of Space, and there is a good chance the greatest (or worst, depends on how you look at it) one of all, AYBABTU was born there as well.