Images Too Big in Firefox and IE

I’m having a problem where my images run over the margins in Firefox and IE (Chrome seems fine).

I have CSS in my theme that prevents images from running over the margins:

img {max-width:99% !important; height: auto;}

However, the plugin seems to make FF and IE ignore that. (I disabled the plugin and it was fine.)

I found someone else who was having this problem, and their suggestion was to add the following CSS:

.pibfi_pinterest {max-width:99%;}

I did that, but still no luck.

Any suggestions?

Thanks.

https://wordpress.org/plugins/pinterest-pin-it-button-for-images/