Greg's method is going to be pretty hard on machine screw sizes. If acceptable (it looks lik you're going to be refinshing the frame anyway) try some heat using a really small tip on an oxy-acetylene torch. Heat the screw cherry red and let it cool off. You may be able to grab the stub with locking pliers and get it unscrewed. Or with a MIG welder, weld a nut on the stub (welding inside the nut), then use a wrench to turn the assembly out. That may be the least intrusive to the finish on the part.
If all else fails, drill them out carefully like Greg said. If you get too far out and bugger up the threads, you can install a helicoil to restore the original thread size.
