To insert an image, copy the link of the uploaded image and paste it between the tags.
If you want it to be in a spoiler, paste it with the tags between the [SPOILER][/SPOILER] tags.
You'll get the image tags when you click at the picture with the mountains, the spoiler tags are located in the upper right corner of the window. When you click at spoiler, you'll have to give your spoiler a name, for example "huge image". Then your spoiler tag would be:
tag code:
[SPOILER="huge image"][IMG]imgur.com/example.png[/IMG][/SPOILER]