Thumbnail size in Photopost and how the featured photo block for the forum is shown are two seperate things. That external forum plugin is just that a self contained file coded a certain way. It is not tied to the core photopost program in any way and serves one purpose. You would need to recode the html and some more variables if you wanted to not use the overlay images. The overlays are one fixed size and the photo is shown as a background so you can not resize them.
try this out
Code:
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