WordPress resizing images

I’m pretty new to WordPress, and have been Googling my way out of most of the issues I’ve had, but I’m stymied on this one. I’m trying to create a portfolio with the Divi theme. The site is:
http://gunga.users.sonic.net/
I’ve created all the projects and sized my thumbnails at 300px x 300px. I’m using these thumbs as my featured image. The problem I’m having is that WordPress will not allow the height of the image to be more than 284px once it’s placed on the portfolio page. It loses the square aspect ratio. When I inspect the element (I’m using Chrome), it shows the natural size as being 300px x 284px. I’ve tried to override with my child theme, but it doesn’t seem to work. I specified the size in Divi booster, and that didn’t work either. Can anyone help me with this? I could just make the images 284px square, I know, and I probably will, but I was hoping to have them show larger when the page resizes on a smaller screen. Also I’d like to understand why this is happening for future reference. Thanks!