This is Frank Thomas, probably in 1954. Behind the scenes photos of Disney animators are relatively rare, so I treasure every one I can find.
Frank animated scenes with Lady, Tramp, Jock and Trusty. The character of Tramp was designed by Milt Kahl, who also animated the character's opening scenes in the film (Tramp sleeping in a barrel, he wakes up and takes a "shower".)
Frank did wonderful scenes with him as he happens to pass by the Darling home and overhears a conversation between Lady, Jock and Trusty regarding a human baby.
Here Frank is animating, what might be his first scene with Tramp, who spots Lady being lectured by Jock and Trusty.
This is that scene, a close up of Tramp looking toward the three dogs,
In the upper right corner of the publicity photo you can see design doodles as well as a size comparison sheet drawn by Milt.
Walt Disney repeatedly assigned Frank and Milt together on the same character. Needless to say the results were always of an insane high standard.