Website showing wrong images

I have a one-page static website. My website is displaying different images than those referenced in the HTML. For example:

<img src="img/About_Us_Graphic.png" alt="About us photo" id="aboutUsPic" style="margin: auto;">

Will be sometimes displayed as the image that's actually

<img src="img/Facebook_icon.png"> 

This happens pretty much randomly. Sometimes the pictures are correct, sometimes they're totally different pictures. And when it's a wrong picture, it isn't consistently the same wrong picture. What causes this? How can I fix it?

My site uses Foundation 5 (not sure if that's relevant). Thanks!