Basically the header moves some pixels to the right/left if you open an Oneboxed image. Size of the image does not seem to matter.
It probably happens because the Scroll Bar dissapears once you click on the Image / you are in the Onebox view or whatever that’s called
This is on Chrome 42.0.2311.60 beta-m (64-bit) but I can repro on Firefox Developer Edition 38.0a2 (2015-03-20) too. Probably Browser-independent.