Like so:
[url=MP URL of item][img]URL of item image[/img][/url]
Will show an image which will be clickable to the marketplace listing.
Simply put, you need to use an image as the link title, instead of words or just the link itself, but to do this you need to use the URL for the image, nested inside image tags for it to appear as an image in the forums.
Quote:
Go to the marketplace listing of the item you want
Right-click the main item image of the listing (don't worry, it won't appear that big when you link to it!)
click the "IMG" box in the post formatting section and paste your link in the box that appears, or, type:
[img][/img]
and paste the url you've just copied in the middle of the two tags.
Now, select/highlight the whole thing including the IMG tags.
Go back to your marketplace listing and copy the entire url in the browser bar at the top of your browser.
Go back to your post and make sure the IMG tags and url are highlighted. If not, do it now.
Now click "URL" box in your post formatting section and paste the url you've just copied in the box that appears.
Following those instructions, you should end up with code in your post that looks like this
[url=http://www.gaiaonline.com/marketplace/itemdetail/100073/][img]http://s.cdn.gaiaonline.com/images/thumbnails/31b74928186e9.png[/img][/url]
Which will appear as a clickable image to the market listing like this
Note: make sure to turn off premium sparkle in your my gaia settings, or when you right click to get the image url, all you'll get is the animated premium sparkle overlay image.