Whether for all your email accounts or per email account customization:


Settings --> (in the right pane) "Signature (Default signature) " and select the EDIT button, make your changes and SAVE

or 

Settings--> <your email account> --> (in the right pane) "Signature (overrides default signature)" and select the EDIT button changes and SAVE



Currently, it needs to be an HTML image (by copying and pasting from a browser) that references an image hosted somewhere else.  We are working on improving this.