If you’re an iOS or Mac developer who creates image-heavy apps, you’re going to want Slender (Mac App Store Link) by Dragon Forged Software. Slender analyzes an Xcode project and identifies unused images, images that lack an @2x counterpart (as well as flags @2x images that aren’t the correct resolution) and allows you to see which files use which images. On sale now for $4.99.
Previously, my solution involved searching through Nibs to look for specific references.
grep -i -r --include=*.xib "image-name.png" /PathToNibs
While this approach has its charms, Slender is more convenient and does much more.