First off:  I'm running CentOS light (If this is a bad choice, perhaps recommend one.  I'm able to switch between distros fairly easily atm) and I don't need a complicated setup, I just need to be able to run a windowed .jar file and interact using keyboard/mouse.  This is for a lightweight system (256MB, perhaps not as 'light' as others) and the application (a network slideshow browser) runs in 1280x1024.  Are there any suggestions, most searches for a lightweight window manager yield something more complicated than what I need.
Best Window Manager for One Application
centos;desktop;desktop environment
You aren't required to run a window manager at all.  I've setup several such applications that are meant to be the only application ever run and they are meant to run full screen and I've never used a window manager.  I always start with a server distro so a desktop and all of it's dependencies aren't loaded automatically.  Then installed X and setup custom upstart/systemd scripts to start X at boot along with a custom .xinitrc.  This sounds like it would fit your situation well.  It takes a little know-how, but it makes for a lightweight system that's very portable.