and see only the URL you've copied and pasted, use the Preview button at the bottom to see a preview of how the post will look to others. The pic, etc., should show up there. Then you can post your message.
And welcome to DU!
This site works well for posting photos:
Tap on Choose Images button, pick the image you want to use from your Photo Library, tap Done. After your image loads, choose the Direct Link option to copy, then paste the link your DU post.