Showing diffs for binary assets doesn't need to include things like "brightness 1%". GitHub currently supports image diffs, they're simply displayed side by side, or on top of each other.
This isn't about showing diffs, it's about merging diffs from two separate changes. The best github can do for that right now is let you choose which one you want to keep, it doesn't let you stack changes to keep work from both committers. For that's you need fine grained explanatory stackable diffs.