As Blogger seems to be insisting on this dreadful new picture viewer, you now have to click the image in the post and then click 'show original' in the bottom left of your screen and then, if the image is larger than screen size, click again for the full size.
Well done Blogger. See why people go to Tumblr?
False alarm, kids! I have been kindly advised by BrittReid how to rectify the problem (see comments).