Near the top of the page, click on 'Upload Images' and upload the image you want to use.

Next, go into your profile and scroll down to the Personal Text & Preferences section near the bottom.

For the Avatar, select 'I have my own Pic' and paste in the full URL to the image.

For the Signature, just paste the picture you want in the Signature field at the bottom

Please remember to keep within the image limits, and don't forget that the forums are case sensative