Just as the title says.

 

Well, you have to fiddle with it a bit. If you want to put your own picture in the avatar (little picture under your user name) you need to resize the image until it's accepted. See the instructions in the stickys. If you want to put a picture with your signature, same thing, have to resize it to the accepted size. If you want to post large, quality pictures it's best to first post them on a picture sharing site like photobucket (or pick another) and either quote the link to the picture or follow instructions on the site on how to attach them to your post or e-mail. Pretty general instructions, I know, but should point you where to start. Look in the 'Appearance & Detailing' section and see what others are doing, or ask the question there.

Or you could take the universal 'short cut' and grab an available teenager and have them show you how.