SlapshotWrangler

Click on your user name at the top right of your screen. It'll give you a drop down menu, your going to want to click on Account Settings. You will be brought to a new screen with various options on a left hand menu. From there click Signature, and the tab will switch to a Rich Text Editor similar to the field you post in. You can then click Insert other media at the bottom, and insert the link to image you'd like to set as your signature. You can use the RTE features like center align etc if you want to move alignments around. 

^ that