Each usergroup has different signature levels. (I believe there is a 15 post minimum before a signature can be added)
The levels are as follows:
Staff/Superfans: 800x200 @750k
Graphics Team: 800x200 @500k
Regular Member: 800x200 @300k
Rookie Member (less than 100 posts): 800x200 @ 200k
Ok, this is pretty simple.
First, go to Account Settings to the top of the Screen:
Now Find "Edit Signature" on the left menu.
The bottom of the next window, check Signature Picture. This is the area where you will upload the signature picture. You have two ways to upload it to the site. You can enter the URL, or just "browse" for the image". notice under the Signature Upload box, it tells you the limits of your signature size. Once you have entered in the location of your signature using either method, click Save Signature. This will upload the image to the site.
Once you are within the Signature size limits, and it accepted your signature, your screen will look like this:
Lastly, click on the link as outlined in the above picture "Insert Signature Picture".
Once you do this, it will enter in the code into your sig box.
Now hit SAVE SIGNATURE. It will display your newly uploaded signature.