Bad distortion in lens, any tips for correcting?

I am a painter and I need to take good images of my work, both for web upload but also for printing postcards etc. I show the above shot of how bad I think the distortion is (pincushion or barrel or both?) and I would like to know the easiest way to correct this.

The painting is perfectly rectangular, and I have rotated the photo so that the bottom is horizontal.

I have Photoshop CS4 and Lightroom 3.2.

Thanks guys!