Optimal Size for Color Section Image

Im using the Enfold theme, and using the Design Agency demo theme. I want to include an image in the Color Section as the Custom Background Image. When I use the desk.jpg image it looks very nice but when I try to upload an image that is 1600X600 pixels (The size of the desk.jpg image that came with the demo theme) it won’t let me upload it. So I cut the image in half to 800X300 and it looks blurry.
So my questions are:
1) How do I make the images not blurry? What is the optimal size to use in the color section?
2) How come the desk.jpg image looks great but my image is blurry?
3) I use photoshop to edit the images, is there any steps there that I need to be taking?

Thanks