display:inline
float:left
is your problem
Floating makes the parents width not be stretched by the child, try placing the image without the float. If you take the float off, it should give you the desired effect.
Another approach would be to make sure you are clearing your floats at the end of the parent element so that they don't scope creep.
Update: After viewing your link Your height issue as displayed, is because the floats are not being cleared.