You're halfway there - you just need to have the image uploaded somewhere first.
Head to
imgur.com, and click the Upload Images button in the upper-left corner. From there, you'll be able to get your desired image online, and they even provide ready-made tagged image URLs for copying and pasting onto the forum, without any need to click the Insert Image button here.
For example, I've just uploaded an image, and imgur places a box labelled "BBCode (message boards & forums)" next to it once it's there. The text in that box looks a bit like this;
Pasting that in here has the desired result;
Hopefully this helps.