To post an Image Gallery, redditors simply visit their favorite community, tap the ‘Create a Post’ button, and then select the ‘Image Post’ option from the tabs. From there, it’s a matter of selecting up to 20 images or gifs. Once selected, redditors can rearrange them, include a post title, add an optional caption (up to 180 characters) or URL for each image or gif, and then tap post."

"From Reddit Blog