I can't believe that level_load(NULL); decreases the fps.
With 3DGS a 3D room (level) actually is the easiest way to achieve what you want.

Although, you could create so called view entities, and move them this way, they don't need a level, as far as I know. Don't know whether you can use ent_morphskin with them. ent_morphskin would come in handy when you want to switch pictures.
The easiest way is to create a model in MED with two flat surfaces, placed in the distance of the radius of your circle of pictures to the model's origin, and load it as a view entity.

EDIT:

Found the chapter about view entities:

http://www.conitec.net/beta/aentities-intro.htm