AHS only allows you to upload an avatar from you computer. You will need to sign up to Photobucket or something to host your signature image separately
Because AHS has many different "skins" for the forum, the signature size limits can change. If your image is too large for a skin, it will appear cropped, but the actual image will still be fully intact where you've hosted it.
http://www.ahsforum.com/forum/faq.ph...q_sig_ava_size - hope this helps some. I like how you've used a pic of Yuko in your sig ^_^