If you're using a web browser to access the site, on the menu bar above the post you should see some icons and font options etc - the final group of icons has 9 buttons - choose the third one - "Insert Image". You can then enter the web address of the image if it's one that appears somewhere else online.
If it's an image you have stored somewhere "offline" you can use the Add Attachment option (top right) or if you're accessing through the taptalk app just add it fro the option that appears at the bottom of the screen when replying/posting.
Use google chrome first of all then you find your image, right click and "copy image address". Chrome is one of the few browsers where it is easy to do this, a lot of the others involve having to download via image hosting sites, too laborious!
Then go to DATM and "reply" to the thread you want to put the post on, right click on "insert image" 7th in from the right (you normally delete the http:// bit first otherwise doesn't work) then copy in using Ctrl + V, or right click and paste. Then "insert image" and then bottom right white text on black background "create post". That's it.