Can I replace text with images on a top link bar? Has anyone done this before in SharePoint Foundation 2010?
If you mean in Sharepoint 2013 there are new delegate controls that enables you to control what will render in the header of the page (in for example the suite bar logo and links).
By creating your own control you can modify exactly what HTML that will be rendered, and hence also display icons.
Zimmergren shows the different delegate controls and how to use them here
I have done it in 2010 too. In my case a little house icon was wanted instead of the global nav link to the home page so I used a little bit of jQuery to find the URL and replace it with the icon.