Not when the ads come from different domains (which is the case for 99% of the annoying flash ads). Adblock stops all requests to these domains. Incidentally, this is also why Adblock is a hit with privacy-conscious people - it mitigates tracking through log file analysis.

no. picture and flash ads are separate HTTP requests. AdBlock/FlashBlock work by not making those requests. You will save bandwidth, but as you have pointed out, it's a drop in the ocean compared to watching a HD movie or even a 10min youtube clip.