Inline content leaves space at the bottom for characters that descend (j, y, q):
https://developer.mozilla.org/en-US/docs/Images,_Tables,_and_Mysterious_Gaps
There are a couple fixes:
Use display: block;
<img style="display:block;" width="100%" height="100%" src="http://dummyimage.com/68x68/000/fff" />
or use vertical-align: bottom;
<img style="vertical-align: bottom;" width="100%" height="100%" src="http://dummyimage.com/68x68/000/fff" />