Looks like there's a large tear that's been welded. Or it might be a crease. There is one spot that might need some shrinking up near the top of that large dent - looks like someone hit it with a piece of pipe.
Otherwise, they're not all that bad. I wouldn't be afraid of straightening them.
To the OP: Pick up a good quality body hammer and an all purpose dolly (aka railroad dolly). Your average ball pein hammer isn't the proper tool for body work. Even something from Harbor Freight would work better than the ball pein you have.
You shouldn't be smacking the fender with all you have, but as suggested, back it up with the dolly and work it easy and slow. It has to go back into shape the way it went out of shape.
Go to Youtube and check out some videos.
