How do seo for website like freepick that post just images

Besides the alt attribute of image tag, use description of images to tell your users and search engines about the images. Treat each such image like a product in an e-commerce website. Use multiple attributes for the image product and create different URLs of such attribute-rich images. These will help in the images website SEO you required.

Yes, @binayjha is right. The ALT attribute will index your images in images.google.com which is enough for generating traffic. Also you can tag images in the meta data. But in general, you don't have to rely on the images themselves. You can create a page for every image, with H1, relevant content, internal links, etc. And let those pages become your organic landing pages. This is the best way your audience will be able to find the images and download them.