Type of adhesive to use to attach seashells to a frame already glued to a wall
There are two challenges to your project as I understand it...


The frame (and maybe even the shells) has a nice smooth finish which glues don't adhere to well.
The frame is mounted on a wall, making clamping difficult.


Abrading the backs of the shells with a wire brush may roughen them up, allowing glue to hold onto them better.  Drilling small holes through the frame finish in spots which will be covered by shells would serve the same purpose for the frames.

As for clamping, use painter's tape.  After you glued a shell where you want it
on the vertical frame surface, secure it with tape to give the glue time to dry.  The tape may not be able to supply much pressure to the drying joint, but immobility alone can help form a strong bond.

With those challenges behind you, the choice of glues is based solely on the materials involved.  I've always had luck with gorilla glue which comes in a gel version for flow-challenging uses.