You need a URL for a pic to put it in a post. Sites like flickr and photobucket give these when you upload to them.
Once your photo is on your flickr or Photobucket page, view the photo and right click and copy the URL.
On your AVForums post, select the insert pic icon (yellow sq with mountain) and paste the code into the little window that pops up, ensuring the http:// isn't duplicated. On your post this code will appear between two s. When you submit your post the pic will be there.
Nice bokeh on your shot :smashin: . I'm assuming this was at 300mm. Is it cropped? For me the beak could have been more in shot either by zooming out a little or panning down a tad. Looks sharp enough but not easy to tell at that size viewing. If you do go for flickr or photobox, upload at minimum large size. Flickr certainly will let you select a smaller size to use for posting here as max is 800x800.