Hello,
The Drop down menu renders the items in the table cells with particular CSS classes. Therefore, the easiest way is referring to the image in the CSS stylesheet indirectly like this:
.<CSS_prefix>CMSMenuItem img
{
float: right;
}
This will set float: right property to all images within the menu elements. And as I believe, this should be your case.
Please note, using of <CSS_prefix> is optional, it depends on your webpart settings.
Best regards
Ondrej Vasil