I've made my first mod but I can't work out how to change the default mod maker preview image into one I want?

I have tried giving the file I want the same name as the mod and putting it in the mod folder next to the default preview image when I upload it, but that didn't work. I tried deleting the default preview image, but then I got an upload error. This is my first time making a mod and I'd appreciate any help, thanks!