Code:
Content, Pictures and Download links visible to registered users only.
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You take the picture you want to use and you let it open in a new tab.
Instead of taking the direct link from the top and using the image tags here, scroll underneath the picture in the other tab. You will see a set of three different codes.
Copy and paste the result codes for the "Forum Thumbnails" set. It has everything needed built into the code, so no need to click buttons here.
Other hosts might call it "BB Code for Forums" instead. Basically the direct link and the HTML codes will not work.
I hope that helps!