<p>How do you go about finding unused icons, images, strings in .resx files that may have become 'orphaned' and are no longer required?</p>

This is no information an algorithm can reliably compute. The inspected program could fetch a list of all resources and do something with them, like letting the user choose from several icons.
Your best bet is probably to search for all references to your resource-access API of choice and inspect those manually. Using
```
grep
```
/
```
sed
```
you might be able to reduce the sites you have to inspect manually by handling all "easy" ones where a simple string is used.