Pictures can be fixed or doctored. There have been numerous reports, supposed sightings, & even footage on Bigfoot, but the fact remains that scientists or people who pursue him have not been able to find a viable trace of DNA or something to prove its existence. I'd catergorize Mothman the same.
Well, there are numerous instances of hair and other DNA samples that have been found while searching for Bigfoot. When analyzed, they are not recognized to have come from any known animal. Does that mean it's Bigfoot? No, since we'd have to have actual Bigfoot DNA to compare it to since DNA doesn't wear a "hello, my name is _____" sticker. But the DNA DOES prove that some type of unclassified animal is out there.
It is incredibly improbably that a large mammalian species, or reptilian, or sea monster, has somehow avoided detection so well. The population would have to be so small as to have probably gone extinct already.