So let's approach this from an analytical perspective. Heck, pretend I'm a judge. I'm looking at conflicting testimony: 1. the van was, and 2. the van was not. What is your point, counselor? You can't argue "it goes to credibility" because I'm allowing the testimony.
After they rendered their verdict, they got all weepy and contrite. Had they been normal American citizens as opposed to the anchor baby offspring of illegals in this area, they might have been capable of something more like mature reflection.
This entire region, (and most of the Border Patrol) is more or less semi-mexicanized and very short of straight-shooters. Typical: René Sanchez, the BP Officer-cuate of the smuggler, who facilitated this whole travesty. Looking for a guy to investigate? Try that scumbag.
Of course they determine credibility but they also use concrete items such as the picture you complain about. That picture is one of those taken at the scene by the supervisor and fuzzy or not, it clearly demonstrates that Compean was telling the truth about something that Davila was lying about. That is the credibility that you are harping about.