If you File>SaveImageAs you can select jpg, bmp, psd, pct, tif. and set the compression level on jpg(lousy). If you File?ExportImage you can select bmp, psd, mov, htm, tif. If you save the file, there is a setting under...Edit>Preferences, ImageWithScene Open/Save.(turning this off will reduce file size) I think this is what you are talking about. This saves a bmp of anything rendered so far, in the .b5 file. Don't worry about this, always SaveImageAs, when you've done a final render. I'd save as a bmp and convert it in post.