[subhumanderelict]

I don't think that it's required, but you're right, it would be nice. It's like marketing; if you want people to try your product, give them as much information as possible. I well-marketed ROM (one where screenshots are provided) will probably have more downloads than one without. It's all up to the person who cooked the ROM. I think it's nice enough of them to share, but by no means do they absolutely need to post screen shots. It's more like saying, "Hey all, I made this, just thought I'd post it here for you all to enjoy. If you don't want to try it, it's all good."