This used to vex me as well, so here's a tutorial that hopefully helps. I think Imgur makes things super easy.
Go to Imgur. Click "new post" in the upper left.
Drag in or upload your image. I used the ERE homepage in this example. Once it uploads, click the arrow in the upper right and then click "get share links"
Then copy the code listed under BBCode (Forums) and paste it into a post.
That's it. One thing to keep in mind is that the forum only allows images with a width of up to 800 pixels. You can adjust the size in Imgur by hitting "edit" or you can do so with your computers editing program. If you are using Preview on a mac, you just open the image then hit "tools" and "adjust size".
A final note -- you do not need to make an account with Imgur to upload pictures.