imgur doesn't work on the forum and doesn't work for loads of users.
Also loads of sites block images not loaded directly, so the "Insert Image" icon tool isn't very useful.
It should work for images on this site:
Go Advanced and attach an image.
There is a box Attach Files. Brings up a window to upload. After it's uploaded the Paper clip icon lets you insert it at current cursor position in the post.
^^ Here is an uploaded image and paper clip applied.
EDIT
Also imgur track people and object to safety script blocking. Their adverts could serve malware