Whenever I use a skin that isn't default for my PHPBB 3.0.4 forum, it keeps saying "Imageset misses “en_us” localisation" in the ACP and they don't let me edit or quote posts. Anyway to fix this?

What skins are you using? They may be non english skins?

Aero Blue, Attriumm, Prosilver SE, and Revolution.

Here, let me help you. Firstly, you download the skin and then open it. Extract the folder that HAVE ALL things in there. Extract it to your desktop. Then open your FTP and go to your skin directory. Drag the folder of your skin from the desktop to the window where all skins are listed (DO NOT DRAG IT ON A FOLDER). Then go to ACP, click to install the skin you want, and that's it.

I did it, the skins pretty much work except for the problem I stated.

You need to rename /en to /en_us. I've had that problem before and it was fixed with that solution. Forgive me if I am wrong.

EDIT: It worked.