How can I add an image to my Tumblr's sidebar?

I’ve added a basic html image embedding code (<IMG SRC=“blahblah.gif” WIDTH=“200” HEIGHT=“100”>) into my Tumblr’s description, which shows up along with the text when I’m looking at it on my own computer. However, it doesn’t show up when other people are looking at it. How can I just add an image to the sidebar myself?

I’ve tried looking at CSS tutorials, but I’m technology illiterate. Nothing I try even appears on the page. Help!