Here's how to attach an image:
Go to imgur: the simple image sharer
Drag and drop the image(s) that you want to upload from your computer into the browser, like so:
Click "Start Upload", and the upload progress bar will appear:
Now look to the right of the image, here you'll see the "Direct Link" to the image. Copy this.
Come back to the forum, click on the "Insert Image" icon, and paste the Direct Link that you just copied:
... And voila:
What's more, you can choose whether you want the image to be a L
arge thumbnail or a S
mall thumbnail simply by adding a lower-case s or l respectively to the end of the file name in the link (before the .jpg).
Once you've added an image you can select the whole line like so:
Then click the "Insert Link" button:
And link it to the full-sized version! Like so: