A screenshot apparently showing Liberty City (from GTA 3 and GTA 4, as well as the various spin-offs and DLCs attached to those games) in the GTA 5 engine has fans running wild with speculation over on the fan-run GTAForums. The shot seems to originate from the online portfolio of someone named Adrian Page, which just so happens to match the name of an artist at Rockstar Games, according to LinkedIn.
The fans are obsessing over every little detail, scrutinizing surface textures and model variances. And while some are skeptical, the consensus seems to be that this is legit. Whether or not it leads to some as-of-yet-unannounced DLC or a spin-off set in Liberty City is unclear. This could be scrapped content, or just a passion project for Page.
In any case, I suggest you enjoy the picture for what it is, but don't get too hyped until there's some sort of official word from Rockstar.