You will have to upload your image to a trusted image hosting site. Imgur and DeviantArt are not allowed.

Imgbb is good to use, like @Lostsalt said.

Copy the direct link from the uploaded image and enter it into your avatar link bar (if you're making it an avatar)

Make sure the image is no larger than 300x300 pixels if so.

If you're posting it on the forum, do the same but post the image link into the forum post. ^^