I had a hard time figuring out how to properly navigate support a little here at first, so for someone else in my shoes, The answer to this question is below. it’s on the userguide page as themeweaver said above. (also, lots of your answers will be in the codex Links on the right side of the support page.)
When using Lush, WordPress creates the following post images/thumbs from the images you upload.
Widgets : 360 x 180
Slider : 1140 x 300
Single : 798 x 360
Gallery : 512 x 256
Your original images should be at least the biggest size you intend to use in both dimensions. So if you are using feature images in the slider they should be 1140×300 pixels or greater. This is a responsive theme so your browser resizes these to fit your desktop.