I think the easiest way to learn to do these things is to find a post where someone else has done it, click on "reply," and then instead of typing a reply, just study what that member typed in order to achieve the desired result. The actual coding they used will be reproduced for you to look at in plain text, at the top of the reply window.
1) First select a picture of the soundtrack cover in google or another search engine;
2) right-click the picture of your choice and then copy the URL link (you'll see what it is when you right click);
3) Then click "POST REPLY" and paste the URL link between the two sets of brackets - in the following pic - making sure to remove the "https://" part of the link;
Saving the photo first to Photobucket is your best bet for posterity.
Simply copying an url from Google will not guarantee that the photo will be available beyond the "now". That means it can be removed by the person who originally posted it to Google and you will have no photo link.