I haven't found a way to delete the garage shot from your profile (not possible maybe). You can only retake a new one as like you said they don't appear in your libraries.
Hi! I made two topics about this haha, so now we have one in each sub-section besides tuning!
It's a small issue but really annoying for people like. There's currently no way to delete them because it only appears for viewing on your profile page, and doesn't appear in your library or gallery. I have two of the same photos (one is slightly glitched so I took another) but I thought taking another would replace the original. Now we learnt that you can have multiple garage shots and scroll through them using R1.
I know Kaz is a perfectionist, and hope he can notice this small (but annoying) issue