ZZ Top, the Prom and Jimi Hendrix

From left to right it's Dusty Hill, Frank Beard, and Billy Gibbons. 

Fun fact, the guitar Billy is holding is the famous "Pearly Gates" Les Paul that was used on most of the albums, and still uses on stage.

"The Moving Sidewalks" hanging out with Jimi Hendrix before their bearded-bluesmen days.  

Only Billy Gibbons played with the Moving Sidewalks. Dusty Hill and Frank Beard joined up with Gibbons after playing with American Blues. The Moving Sidewalks made better music, American Blues released albums on a medium-sized labels (Kama Sutra & Uni).

Billy Gibbons from 1969

Dusty Hill and Billy Gibbons 1973