It appears to hold the right size on mobile but not on the desktop bowser
Mobile is down-sizing the image to fit in to a mobile container.
I see where your problem is happening; you're using wallpaper sized image for a category button. Resize your forum images so they will fit in to the expected space. These images will be displayed next to the forum name on the Main Index page. If you use a wallpaper-sized image here, you'll also be using a large file that needs to be downloaded by every visitor, every time, who doesnt already have it in their cache. that can get very pricey on bandwidth for both you and them. Your high-trafic images should be optimized.
Your forum images do not need to be squares, such as 100x100. On several sites that use forum images, I've seen sizes such as 120x80 that work out nicely.
Further advice that I see you're already following, but worth mentioning for others, is to select forum images with high contrast or images that do not require lots of detail to understand when their size is reduced to a thumbnail button.
