If you’re just manually adding the graphic in the memo:
insert the graphic
select text on either side of the graphic (to include the graphic)
Choose Create > Hotspot > Link Hotspot - get the dialog box below
Enter the URL you want the graphic to go to
Ta Da:
Final product:
If you want to make this a part of your signature or something then the particular HTML tag you want is the ancor tag. So your signature would have something like