You need a URL as far as I can see.
Store them online with something like Google+ or photo bucket.
Copy the URL and paste in the image box (Windows version anyway).
Might be different with Tapatalk as it uses the attachment tool which tend to show a clickable thumbnail.
It all done in virtually 3 clicks, browse, select size ( i use 800x600) upload then select one of the forum hotlink "copy to clipboard" buttons.
Then right click here and paste....done!