Can anyone tell me who the guy with the yellow and black mask is?
He's in the top right corner. Directly above Batgirl and to the right of Superman.
Image source - Harley's Joker
The original "Joker Trophy Wall" artwork is from "legendary" comic artist Brian Bolland drawn in 2005 under commission. A gallery posting about the origins and content of the original artwork is found here. The artwork is not based upon any story in the DC canon.
The cosplay image was created in 2015 by photographer and cosplayer Anthony Misiano as a homage to the original artwork, and of course for the sake of his own publicity and sales. A few articles about the cosplay image can be found here and here.
To answer the question itself, according to the description section in the artwork posting, the unknown "trophy" character is "Animal Man". And interestingly Brian Bolland created the cover art for the 1998 revival "Animal Man #1" (and is likely the reason that Bolland chose to include him in the 2005 Joker Trophy Wall artwork). For reference, in the below comic cover, note Animal Man's triangular goggles in the mask, and blond hair with stringy bangs on his forehead.