Help with theme

[Valiant]

First off, wanna apologise to admin cos I didn't know where to post this.

If you take a look at my gallery at www.selfharm.tv/gallery I want it to resemble the forum at www.selfharm.tv/forum as much as possible.

Here's my question, basic as it might be. Those cells with the pink background, how do I add a background image to show up instead of the colour? I've tried using CSS without much luck.

If you know how, please if it's not too much trouble post a step by step or just the code and file.

Thankyou in advance!