Image in a Custom Block

Can someone tell me the correct way to insert an image via custom block in OP? I do have Full HTML turned on but when I save the block it renders wrong in the browser. Is there a setting I am missing?

For example, this code:

<a href="http://www.decaturfarmersmarket.com><img src="sites/default/files/ads/decatur_farmers_market_Color.gif"/></a>

Gets rendered in FireFox as:

<a decatur_farmers_market_color.gif="" ads="" files="" default="" sites="" href="http://www.decaturfarmersmarket.com><img src="/>