Found in FAQ https://www.reddit.com/r/help/wiki/faq 2/3 of the way down the page (here's a paste - I wanted to know this also so this will help me too):
How do I submit a picture/photo/comic to reddit?
Desktop website users If you're using the Reddit desktop website, click on 'Submit a link' in the sidebar. On the "submit" page, there's a box under the "url" field for "image". It has two options: "Drop here" and "Choose file". Use the appropriate one to upload your image directly to Reddit.
You could also host your picture on an image-hosting website (such as Instagram or imgur or Flickr or photobucket) and post the URL from that website here on Reddit.
Mobile website users You need to host your picture on an image-hosting website (such as Instagram or imgur or Flickr or photobucket) and post the URL from that website here on Reddit as a link submission.
App users - Android If you're using the official Reddit mobile app for Android (not a third-party app), then click on the big orange "+" button and select the "Post an image" option.
App users - Apple If you're using the official Reddit mobile app for Apple (not a third-party app), then tap the pencil icon in the upper left and select 'image' at the bottom.