Well, you can try that one:
[Code]
IMPORTANT NOTE:
I set on each @media Part a different color, so you can view very quick on wich width the color of the background is changed including the size of the images. It's just for a internal controlling.
For the final release in public, remove that colors by simple delete the line (example: background-color: blue;)
Pls. check my CSS on other areas on the board, if something has changed to what we would not change. For that check, the background colours are helpfully to, because you will see it immideatly

.
On the last Screensize (320px) I decide to remove the thumbnails, because they would be so tiny to fit it on the page, that it make no sense for me. But you can set your own dimensions if you like. Names and posting will still be there, but not looking perfect on a small screen because the text have no space.
If you give a bit of more controlling around that part, like hiding the full area of "Posters of the week" to in the last step (smaller than 320 pixel), you need to create a new CSS defintion around this part like a
[Code]
I think the posters of the week part is in the footer of the layout. There must be a line with "<h3>Posters of the week</h3>", so place the
<div class="posterofweek"> before and than after the "</table"> of that are the
</div>.
This is just a quick fix from my side. There are much more ways to handle thumbnails / gallerys in a grid, controlled over CSS, but for that you need to code some more and lot of things will be complicated with phpBB (or general if you working with Databases in the Background)
If you need any more help, let me know. Also over PM.