Read the User Agreement regarding signature pictures: http://www.aquariumadvice.com/viewtopic.php?t=15094
Use Photoshop or other editing photo software to resize your picture to the size limit stated in the User Agreement. Then put the picture in your gallery.
From the homepage, click on "Update your Profile". Go down to the signature block and use the img tags, like krap101 mentioned. It helps to have 2 windows opened; one for your profile and one for your gallery. Right-click on the picture in your gallery and copy and paste the image location (Image properties>Location) to the sig block between the img tags.
Scroll down in your profile to "Preferences" and click "yes" for "Always attach my signature".