Would you say hers photos? No. You would say her photos... or if it's already clear what you are talking about, you would say hers without specifying photos.

So you first have to establish that you are talking about photos by saying half your photos and then you can say hers.

I remember [that] you sent me a photo with half your photos and half hers

And you are right, collage or photo-collage would be a better term to use for the composite photo.

If you would like some technical terms, her is an attributive adjective and hers is a predicate adjective.