Image "compression" is when the quality declines due to bad formating, resulting in clumps of color that are noticeably separated from other areas of an image. I see a lot of it in the sky. When I get that, I use a blending or smudging tool to smooth out the edges or eliminate the compression all together (if possible). [link]
Best of luck. And I'm sorry for not responding sooner; I come and go on Deviant Art.