Registering

Great. Now please go to the thread that gives the badge. You can find it in my first comment and follow wykoon's steps:

Add pictures, screenshots, media or documents to your post

Click the second icon of your comment box that is circled in red in the picture. Then your phone should allow you to pick the right screenshot in your images files and upload it on the discussion. Screenshots are probably in a file of their own in your phone, separate from actual photos.