Are these walls holding up my roof?
Yes......................................................

Let me elaborate... FOR SURE!  

Given the picture is accurate and the (large) kneewall hits the joists the kneewall is functionally taking some of the load from the top roofing members and relocating that load to the floor below.  You could replace the kneewall with a header of adequate size.  But given you keep the same joists framing below you would then have to pick the load of the header at the 2-3 spots that it runs to the first floor and then carry the load to the ground.

We are actually building a large 1000 sq/ft garage right now with the exact same layout of the picture below - I could provide some pics this weekend.  We talked to the architect about moving the second floor knee walls close to the edge but we are stuck at about 3.5 fee on each side - so making storage.  

If it was a continuous peak to the edge and the kneewall was an actual kneewall (2-3 feet in height) then you may be able to move it.  But as the drawing stands and factoring the height, the space from outer walls, and that the joists are on them I am 99.9% sure they are load bearing unless there is a lot of metal involved.