I've been getting my car up on jacks often. I've noticed all these places where someone has jacked the car up and bent what i believe to be the subframe. What should I do about these pieces?

Drivers side front

This is harder to see but it's bent in the middle like an upsidown U(front passenger side)

 

Ya could pull it, or weld another piece to make it flat, just make sure ya rust-proof it --I have the same thing

 

is this a big problem? Would the "right Way" be to pull or weld a new piece on? what thickness should the new piece be?

 

Well my word may not be the best word out there (? everyone else) but I think you should pull it, if you decide to do anything about it... looks like it may have bent the frame a bit? I wouldn't do anything if I had a choice and it was my car
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I'll look at my car here tommorow and see how hard it would be (gotta sand it down again for final)

 

Nothing to worry about. It is caused by people jacking car up without using a block of wood to spread the load. Sharp edges on jack pads are usually the culprit.

 

I think I'm going to weld in some plate. What thickness should I use?

 

I think you are looking for more trouble than its worth. Especially considering where you are looking to weld. I would leave it be, its not a big deal. You are only going to introduce rust into the scenario.