You may want to style

div div img

{
width: ???px
}

together with the classes, you can add a bunch of classes (I counted 9!) to the control:

<dnn:TREEVIEW runat="server" rootonly="false" level="child" id="TREEVIEW1" includeheader="false" nowrap="true"/>
please check the skinning doc. 

Looking at the skinng doc the tree control is very powerful, or complicated, depending on you standpoint, thus requires a lot of experimentation to get what you want. Just like the solpartmenu you can write a thesis about it when you want to explain all the nice things one can do with it.

Sorry, I can't be more helpful, I haven't had the opportunity to use and skin this control myself.

Peter