TMZ has obtained a photo of Aaron Hernandez throwing up gang signs often associated with the Bristol Bloods street gang in Connecticut ... while dressed head-to-toe in red clothing.

The photo was taken approximately 6 years ago, when Aaron was 17-years-old ... and a senior at Bristol Central High.

Aaron was long-rumored to be a member of the gang ... and now, with this photo -- combined with the "blood" tattoo spotted on his hand during his arraignment -- the doubt has seemingly been erased.

23-year-old Hernandez has been charged with the murder of 27-year-old Odin Lloyd. He's due back in court later today.