Although it allows you to link to something on the internet it's not a nice thing to do unless the site specifically says it's okay. Right click on the image and save it to your computer and remember what folder you saved it to or just save it to your desktop.
On the left side of the forum just over the Knitpicks ad is a link that says "user settings." Click that then go to "edit avatar." The info for maximum file size and kb is listed under the upload from your computer link.