Make a 24x24 px bitmap (*.bmp !) with the emblem, then go to your RO folder, create an "emblem" folder, put your bmp file in there, after that start the game, press ALT+G (as the guild leader) and click on the Edit-button to change your emblem.
- It must be a bmp-file. How many colours you use, doesn't matter.
- The RGB colour "255, 0, 255" (it's pink) is the transparent colour ingame.
EDIT: The faq is wrong in one point. In fact, it's not obligatory to make the bmp file 8-bit.