HI Meinmk5
I usually create an Images folder to start wit and plonk them in there and then take a look at this article where RIck discusses Relacive Paths
http://west-wind.com/weblog/posts/269.aspx
I'm not in favour of hard coding url imge url's into the web.config because if you need the same image in two seperate places your copying the code and then modifying it for the same image.
Anyway if you want to go for the web.config route try this.
<add key="homepageicon" value="~/images/homeicon.gif"/>
Then you just read the value of homepageicon using the congiuration manager - along those lines.
Give me a shout if that doesnt help.