Displays the actual content of the posting?

See http://www.fabulaemarrii.com/guildnews2.php

I'd like to replace the link with the content of the actual posting itself (that is: 1) Name of poster 2) Screenshot 3) Body text 4) Link to big graphic).

Is this possible?

It's possible. You'd need to actually do some queries similar to the recent topics islands but grabbing the body as well. If you're familiar with PHP at all you take a look at the cache_builders/post_island.php that will give you a bit of a start.

I've done a bit of PHP before, but this code is way above my head I'm afraid

Hoping that some PHP guru wants to do the same on their site, I remain...

What I need is basically a portal page that can be integrated on the rest of the site, without left and right columns, with postings from one particular forum.

Maybe the code should be based on portal pages instead of custom islands?

Eventually as more people upgrade we should start seeing more addons and customizing floating around. This might be something best suited for UBBDev.com as you're probably looking at a full new script as you're thinking now. It sounds like the main portal page could be a starting point for you as that pulls posts from designated forums.