Bloody GIR is a little hidden secret placed into certain episodes of the series by the crew of Invader Zim. It features a picture of GIR covered in blood, hence the name. To see them, one would need a player that can do a frame by frame, and would need good eyes to spot the frame that has the bloody GIR. For example, in Bad, Bad Rubber Piggy, towards the end, if one looked at the final pig as Zim throws it into the portal, they would find this image. Another example would be in the opening sequence of episode 22 (Mortos Der Soulstealer/Zim Eats Waffles) when the pipes come flying out of Zim's house
I havent seen him on the show yet and I really want to, I just learned about him recently over the net.