Inserting a linked image such as an album cover from the web is straightforward.
1) choose your jpeg image - copy the url from the address header bar. Amazon or Wikipaedia is a good image resource for album covers. You will need the specific url of the image itself, so right click and open image in new tab helps here, or else search on google and press view image on the one that you choose.
2) In the post reply box, click on the button with the image of a mountain in it (insert/edit image button). The insert/edit image dialog box will open. Paste into the Image URL box labelled "Source". You should then see a preview of the image. A very large image will mean you will only see a small part of it as it's not scaled in the preview box.
3) Press the blue OK button at the bottom of the dialog box. The dialog box will vanish and your picture will appear within your post.
If you wish to post one of your own images then you will need to use an image hosting website such as Flickr, Imgur etc..
You should copy the "Direct" image URL for the image you wish to post on the forum.
In short: In the Insert/Edit image dialog box (mountain picture button), paste the URL into the "Source" box and then click OK. The image should then appear in your reply. Then just click on Post Reply