Custom item asset

I was wondering if it was possible with sponge to have custom item assest as in that an items own image without trying the vanilla ones

Images are not stored on the server with the exception of some mods. Even then any modifications you do does not get sent to the client

so the only way there would be to do it is to have a custom resource pack they have to use

Yep. You could send a resource pack over the server but the client needs to accept it