Where you bought them makes no difference.
If images just over 700mb aren't working (and you've tried different image sources to eliminate bad shares) then it might be crappy media or your burner won't write 700mb+ (I think a burner issue is unlikely)
The cd's (if they're crap) might work for smaller images, but if they aren't working for 712mb then it's a process of elimination to work out what the problem is.
You should be fine burning @ 4x speed...I've never had any issues with playstation games burned at this speed.