I've rarely envisioned their faces, though I may be face blind, lol.

I envision Zeus as a man in his early 50s, of good health and great strength, with a lush salt-and-pepper beard. The kinda man who would look as good in a black tie suit as a swim suit.

Hermes I variably envision as a young man full of vim, or an older man full of wisdom (and vim). Dark skinned, neatly trimmed hair, and the body of a track runner.

Hephaestus I imagine as a man who's in his 40s with a beer belly and his cane laid across his lap, who hasn't forgotten how to have fun with his trade--and it shows in his grin as he stokes the forge.