Some forums have limits on the dimensions and file size of avatars. It really can be a money issue, since web host services generally price their hosting plans based on hard drive space allowed on the server and the amount of data transfer. Data transfer refers to the volume of data that must be transmitted from web server to web visitors computer; of course, it includes all images included the web files comprising what the website visitor sees on their computer.
Graphics optimization is kind of a balancing act, or striking a balance between image quality and file size. Now, I'm not advocating that everyone go buy
Optimizing Web Graphics software, which is specifically designed to optimize images for web use.
Again, merely resizing an image to the actual size of the avatar or intended signature block image instead of taking a larger image and forcing the web browser to reduce the size is major step in avoiding bloated graphics. Also, digital photos straight from the camera can be huge files. The comment, "One picture could have originally been 3-40MB, but sick PhotoShop or Fireworks on it and it will be 18.2KB" was right on point.