[*]Are they hydraulic or cable disc brakes? Sounds like they either need bleeding (hydraulic) or the pad's adjusting closer to the disc (cable). Though it might also be oil on the disc (if you spray lube onto the chain/cassette), clean the disc's with a solvent & sanding off the top layer of contaminated pad
[edit] just seen the piccy, they look like cable adjust. Check the pad clearance, you'll need to adjust the brake up at the lever for the cable actuated side, & manually screw in the inner side (nearest the wheel) by means of an adjuster on the caliper or an Allen key (depend on what model they are)