Can I hang a 100lb punching bag from this joist?
I wouldn't call that a joist, I'd call that a tie. A joist is something supporting a floor, designed to take vertical loads in bending. A tie is something designed to take tension loads, in this case that tie is there to stop the roof spreading sideways at its base.

Now, just because it is a tie doesn't mean it can't also take some vertical load. In fact, it looks like there may be a vertical member running from the roof ridge down to this tie, which in all likelihood does bring some vertical load down onto the member.

So the important question is "What spare capacity is there for vertical loading on this member?". If you put bending load into a member, you reduce its capacity for axial force. So in order to answer, we'd need to know:


the member size (breadth, depth and length; easy for you to measure)
the wood type
the current axial load on the member (very difficult without knowing the whole roof structure)
the current vertical load on the member (very difficult without knowing the whole roof structure) and its position on the member


We really can't answer all of this over the internet. You could employ a structural engineer if you want, though this might be overkill. But, never fear, I have a solution!

I'd be happy to expect the walls to be able to take an extra 100lbs. So what we want to do is, instead of risking overloading the tie and destroying your roof, put in a new joist specifically to take your punching bag. You can take a flyer and just buy one if you want, or you can try and do some engineering calculations for it, to make sure you get a big enough one. There are a few questions on here about joist design, if you search for them.