You seem to be talking about ranks like $5+ donator and crabby etc.
Click on the section titled "Forum Ranks" in the link that BLEUSKITTLE posted for all the information about how to get one. Most of the ranks are given out to staff members and some you can win through competitions or helping out the site in some way.
Just make sure it's within "a maximum of 350 pixels in height including all quotes/text/pictures in it and a maximum of 650 pixels in width with all text/quotes/pictures. "
To check the pixel size, right click on the image and view the properties.
You posted a certain amount in those sections, that's why you have it. However they will not be there forever and you'll have to keep posting if you want to maintain it.
Those are participation miniranks. They're granted to you if you actively post in a specific section. To see which section the minirank is related to, you hover over it with your mouse. If you're curious about other miniranks, check out the minirank directory in the user center.