My colleague *may* have forged data. Any advice? We are about to submit a revised manuscript for publication in a prestigious journal. This is a team work and one of my colleague performed most of the molecular biology experiments, including many electrophoreses. 

I just noticed that on one of these electrophoreses, if play around with the contrast etc., some bands are surrounded by a rectangle with a different background as if the image had been assembled. 

When pointed this weird detail (without making accusations) and asked where that came from, he said something along the lines of "wrong image, my mistake", and sent me a different gel later on (without this weird signal).

Now, I can easily imagine how one may accidentally mislabel an image, but I have a really hard time imagining how one can accidentally copy-paste signal from one image to another. I am now concerned about the validity of all the work that was performed by this teammate. Not only that experiment, but also graphs etc.; while I have no further evidence of weird results, I am now suspicious of anything that was produced by this person.

The deadline for sending our revisions is in a few weeks, and the experiments performed by this teammate are both 1) too tedious to be repeated within the next few days, and 2) too important to be removed from the paper.

I am afraid that if I bring this up with our PI (who is good friends with this colleague), and if my colleague finds a way to talk himself out of this, I would be seen as the "team breaker" and have a really bad time for the rest of my graduate studies in this laboratory. 

How should I approach this, and with who?
Talk to your PI and/or colleague. It might be wise to start with something less confrontational than the "I think [Colleague] may have forged data", however. Maybe "hey, I still want to understand what was going on with that figure. Perhaps other figures might be affected too? What do you think?".

Best case scenario, there is a convincing answer that can calm your nerves. There are a whole slew of other worse possible scenarios that can follow from this discussion, but they're all better than finding out after publication that your suspicions were correct. As for the revision deadline, it can likely be extended if need be. It'd be up to the editor, but I imagine few prestigious journals would say no to a reasonable extension request to double check something important.