kb is k. It's just a shortened version of kb. It's how much space the image takes up on the computer and on the board. If you had a 500k image, then the person loading the page would have to load that image for as many posts as you had in that thread (the page they were loading). Therefore, if the image is very large, it slows down peoples computers a lot, making the page load very slowly. This is what Petrie is trying to avoid.
The avatar I have right now is 17.22k and has a resolution of 120X73, which complies with Petries requirements. You must do the same when creating a new avatar.
This is my third post, so when you load this thread, you're loading that image 3 times, which makes 51.66k you're having to load, just in my avatar! Well, most people get around 2,000kps, meaning that every second they can load up to 2,000k. However, if you use a 56k modem, then you can get a maximum of 56kps, meaning that it would take almost a whole second just to load my avatar. My avatar is by no means all the stuff you'd have to load either.
Creating the avatar is only half the work. The rest is ensuring that your avatar complies with Petries requirements and doesn't slow other peoples computers down.