To add an image to a channel, click the icon above the image with a square and a plus symbol. A dialogue box will appear. Check the box of the channel you wish to add the image to.
Images can be added to multiple channels. You can also add others images to your channels to collect them. Just use the post button on any image.
Keep in mind: Images in channels will appear in the order added. If you are creating a presentation, add them in the proper order. If you do need to rearrange your images, this can be done from the mobile app, free for iOS and Android.