Gareth is right: at this wide angle you will get the shadow of your own lens on the picture when using the internal flash. Happens quite quickly, and there is nothing you can do about that. Just zoom in a bit more, or don't use the flash. No other alternative I am afraid...
Cameras: Canon EOS 7D, Canon EOS 40D
Lenses: Canon EF 17-40/4 L USM, Canon EF 24-105/4 L IS USM, Canon EF 70-200/2.8 L USM, Tamron SP 150-600/5-6.3 Di VC USD, Canon Extender 1.4x II
Want list: Canon EOS 5D Mark III, Tamron SP 90/2.8 VC USD Macro, Canon EF 500/4 L IS USM, Canon Extender 2.0x III