If an image is less than 250x166 pixels, it will not show up in the facilities list. It will still appear in the facility entry’s slideshow, however it would still be very small and isn’t recommended to be used. An image really needs to be at least 720x480 pixels in order for everything to work properly in the module.
With its interface being responsive, that is the ideal image size to keep all of the formatting/sections in their correct places. Images less than this size are what can cause things like the Features list to become slightly misplaced.