For every other contest the ballot shows the voter chose the Republican. Isn't possible the voter made a mistake on the form and the sticker was the fix or that it was altered by the people looking for alterations to the ballots?

All the picture show is that there is a sticker over the Kerry block. It does NOT show that an election worker put it there, that an election worker then blocked in Bush, or why and by whose hand the sticker got there in the first place. Here, for instance, the ballots are fed directly into the scanner by the voter, attended by a poll worker who does not do anything out of the sight of the voter, just assists with feeding the scanner if necessary and makes sure the voter is only putting through one ballot.
It really means nothing at all by itself.