It doesn't matter what page I'm looking at,.. as soon as I add a subpage, the styling of the root menu item that has been given a subpage gets overridden by inline styles that override my background image.

Root menu item in question; culprit style highlighted in yellow. (Root menu item and subpage not selected)

 span title="Asset Management" class=" mi mi1 id58 root" id="dnn_dnnNAV_ctldnnNAVctr58" style="background-image: url(/images/menu_down.gif); BACKGROUND-REPEAT: no-repeat; background-position: right 50%;" _events="[object Object]" ns="dnn_dnnNAV_ctldnnNAV"

Root menu item selected:

span title="Asset Management" class=" sel  bc mi mi1 id58 root" id="dnn_dnnNAV_ctldnnNAVctr58" style="background-image: url(/images/menu_down.gif); BACKGROUND-REPEAT: no-repeat; background-position: right 50%;" _events="[object Object]" ns="dnn_dnnNAV_ctldnnNAV"

sub page selected:

span title="Asset Management" class=" bc mi mi1 id58 root" id="dnn_dnnNAV_ctldnnNAVctr58" style="background-image: url(/images/menu_down.gif); BACKGROUND-REPEAT: no-repeat; background-position: right 50%;" _events="[object Object]" ns="dnn_dnnNAV_ctldnnNAV"

Root menu item with no subpage: this root menu item selected

span title="Services" class=" sel  bc mi mi2 id59 root last" id="dnn_dnnNAV_ctldnnNAVctr59" _events="[object Object]" ns="dnn_dnnNAV_ctldnnNAV"

This root menu item not selected:

span title="Services" class=" mi mi2 id59 root last" id="dnn_dnnNAV_ctldnnNAVctr59" _events="[object Object]" ns="dnn_dnnNAV_ctldnnNAV"

Just to be clear, I do not have an image called "menu_down.gif"  in my images folder.

If you are styling the menu yourself, it would be better to use these classes: "mi mi1 id58 root etc."

In your example they all seem to be what they should be.

"sel" for selected and "bc" for a parent of a sleected page