Uploading images directly is a no-no! Here are the steps to upload images properly to the slack channels
- Click your name on Direct Messages section of sidebar.
- Upload image.
- Open the uploaded image, Create external link from the menu.
- Copy and paste external link to appropriate thread.
Unfortunately, there is no native way to do it in mobile devices. Make use of some 3rd party image sharing software such as Gyazo (download for iOS , Android)