I have been taking an interest in the Pegasus UIN list in recent weeks, wondering how such a daunting task begins in the first place. But it's one of those projects where just about anyone can find something that helps.
I cleared as many missing Atlanta Braves as I could, but another resource I have I'd almost forgotten about. When I renewed my interest in the hobby a few years back, someone on eBay was selling data CDs with photos of every player from certain teams and eras; his name was not on the product, and I regretfully do not remember.
But one of them covered the Oakland A's from 1968-97. A couple of these images came friom that disc.
1973 Rob Gardner (from okcochise)
1972 Mike Kilkenny (from Bottomms cards)
1977 Jim Umbarger (from disc; was originally Lexibell?)
1988 Ed Jurak (from disc)
1991 Brad Komminsk (from back of 1992 Score card)
Hope these help!