I'm not sure I understand the question. The literal answer is: you upload pictures! If you meant, "how do I get people to upload pictures?" then I suggest you have to offer a service that people want, and either advertise on social media, or hope that word-of-mouth will get around.
Personally, I'm not sure why someone would want to use the site?
As I am getting you wanted to ask how can you acquire more visitors, users for your website- Right? You can target keywords image, image hosting and other similar keywords, You will get results.
One more think I would like to suggest, even though you have good call for action, but you can enlarge your home page, can put some more pics, add effects!