There are a couple of things you can do to maintain image quality. Our media manager automatically converts the images you upload to a .jpg format. That may be one reason you are seeing a change in quality. If you upload your images as .jpgs then they won't have to go through that conversion process.
Also, note that when you are selecting images to use in a course, there is a pull-down tab that allows you to use the "unison optimized" or the "original" version of the image. Selecting "original" will allow you to use the original jpg (just watch out for possible sizing issues.).
Another option is to upload your images as .png files. Just make sure when you are uploading the images to the media manager, you uncheck the "convert pngs to jpgs" check box. There is more information on how to do this on our wiki here and here.