The JPEG Quality on the IRender Image Tab lets you set the quality (vs file size) of saved JPEG files.
Settings at the left side of the slider create files about 1/2 the size of full-quality JPEG files, but for some images, (especially images with large areas all in the same color - such as the ground plane in this image), the compression "noise" will become objectionable. We recommend the highest quality JPEG setting. (This is the default setting.)
Files saved in .PNG format have no compression loss at all, but may be larger.