I use an app called 'Phupp' and you upload your pictures onto that app, then copy the URL of the image and use it in the Tortoise Forum app. To use it, you press the '+' sign, and choose image link part to upload your image. It always works for me!
I don't receive an error. The pictures just don't show up on the thread. The only way to have it work is if I use photobucket. And I cannot get push notifications either. When I press it in settings it doesn't do anything.