I would say the bottom bracket area at first guess. the bike looks pretty new if it is in warranty take it back. I had a bike a couple weeks ago that sounded just like that and this it what it was..
the seatpost was such a tight fit that the owner had to twist and beat and pull on it to get it to move up and down. Which in turn flaked off little shards of the seatpost that dropped down into the spindle bearings and cups. took it apart removed the chunks repacked the bearings and a reemed and cleaned the post and seat tube
to service yourself you will need some tools if you take some close up shots of your bottom bracket and crank we can recommend the prper tools for you
There are two kinds of people in the world, "Those who help themselves to people, and those who help people!"
I experienced something similar.
I'd just bought a new bike and had probably logged a little less than 300 miles on it before I started experiencing something similar to what you're experiencing (if not the same exact problem). I noticed that while pedaling in certain gears, there would be a moment where the chain felt like it suddenly went slack (like the chain fell off the gear) but then literally a fraction of a second later, i would feel normal resistance again and it would feel normal.
I bought the bike from a shop several dozen miles away from my home so first I took it to the LBS (local bike shop) and they gave me some runaround half cocked theory for what was causing my problem. Needless to say I was 20bucks short after the visit and still experiencing the problem. So I took it back to where I bought the bike in the first place to tell them about the problem. They diagnosed and repaired it in a matter of minutes.
The mechanic explained to me that after riding a new bike around for awhile, the cables will stretch a bit. Since the cables have stretched out a bit, the chain will jump off the gears once in awhile during pedaling. For me, this would happen when the chain was in certain gears, not all of them. They adjusted how tightly the cables were and after that, the problem was solved. I'm pushing 700 miles now, 2 months into having the new bike and I haven't experienced the problem since.
Hope this is of help to you. I suggest you take note of what gears you experience this problem, then take the bike to the shop and tell them you want to put the bike on a repair stand and crank the pedals by hand while the chain is in those problematic gears. While pedaling, watch the chain. Is it "jumping" off the rear gears and then settling back down again? If so, then you're experiencing the same problem I did. Adjust the cables and you'll be set.
Hope this helps
by the pictures I would start by servicing the bottom bracket. looks like uneven gap around spindle, check for any side play.
you will need a crank puller, a pipe wrench will remove the lock ring and a cup wrench or a large adjustable will remove the cups. replace the bearings grease well and reassemble
read the tutorial on this site for help
There are two kinds of people in the world, "Those who help themselves to people, and those who help people!"
thanks guys.
if the bottom bracket really does need maintenance, how long would I have to get around to fixing before it really dies? (maybe in terms of miles?)
I guess, is it generally urgent?
the longer wait the more damage you will do. take apart now inspect the spindle and cups. they may be ok. and just replace the bearing. if you keep riding you will have to replace the whole bottom bracket
There are two kinds of people in the world, "Those who help themselves to people, and those who help people!"