Photoshop Elements :: Wood Art Items - Touch Up Background In Photo

I use a light box to take pictures of my wood art items. The walls of the light box are made from foam core boards and light wooden frame for increased stability. Even though I can place a poster board for background color inside the light box and curve it up the back to create a seamless surrounding, parts of the corners may get in the picture.
 
I have recently purchased Photoshop Elements 11 and am in the learning curve. Can I touch up the areas of the light box corners that show up in the photo? This is probably done in Editor. I have used another "old" editor Microsoft Picture It! Publishing Platinum 2001 for years where I would use paint to touch up the background or cover up something in the background. But it was difficult to get the background colors to match even if the color was white.