I would first look at putting in the absorption panels. Twelve 1' x 1' panels are not a lot. A standard size 24" x 48" panel is equal to about 8 of those. It's better then nothing, though. It may help cut down some of the slap echo. Check out GIK Acoustics if you are interested in larger size absorption panels for a very economical price.
Once you have the absorption up, be very careful with diffusion. Certain types of diffusion panels, such as QRD or skyline, can cause cancelling of some frequencies and boosting of other frequencies, especially when they are mounted very close to you in a very small room. If they are just generic reflectors (such as the pyramid style diffusion), then it may be less of a problem.
Also, I noticed that your room is almost perfectly square (12 x 13). A square room is actually the worst acoustic situation because of the equidistant walls creating standing waves in both directions. A better room would be more of a rectangle shape. :). Though sometimes we have no control over this.