Board Thread:Support Requests - Getting Technical/@comment-24956068-20150823200501/@comment-4731443-20150823204136

I think this would solve your problem: .pi-theme-YOUR_CUSTOM_CLASS .pi-image-thumbnail { max-width: 250px; height: auto; } max-width will make it so larger images are scaled down, but smaller ones won't be scaled up. height: auto makes sure that the ratio is kept between width and height.

If you're not yet familiar with custom CSS for portable infoboxes, see Help:PortableInfoboxes/CSS.