Single post in A Somewhat Extensive Guide to BBCodes

I'm sorry if this was already covered and I somehow missed it, but I've drawn and properly sized an Avatar image and I have it not only on my DevArt account but also on my computer. God I must be getting old, but in the 'Change Avatar/signature' section of the 'Farm' Tab I don't know what to put in the text box below the default avatar image. I tried sourcing from DevArt and my computer but I just got an error message. I know it's not he format, it's saved as .png. Help, please?