Adding your own art can make your site feel like a gallery or portfolio, showing your viewers what you like to make. You could also add other people's art that you like, but you'll want to either check if they allow usage of their artwork for it or contact them directly to ask if they dont have a post or set boundary listed anywhere to make sure you have the artist's permission before adding it. If they ask for credit, then add credit with either text near the art or provide a link.
Photos can add to the overall aesthetic of the site, giving people something to look at. Similar to art, you can add your own to showcase your own photography skills. If you have a specific subject you wanted for your theme then you can add pictures of that subject.
With their small size and fun designs, blinkies are cool little gifs you can add to your site for people to look at and enjoy. You can find a bunch of blinkies on Neocities, Tumblr, SpaceHey blogs, etc. or you can make your own with sites like Blinkies Cafe or from scratch.
More Info Examples/My CollectionSimilar to blinkies, they are small and fun to display in collections. Many people have both blinkies and stamps, and they can be found in similar places.
More Info Examples/My Collection