JPEG format is recommended for any files that you intend to view but not edit, because they have the most effective compression algorithm, i.e. you get the smallest file sizes.  If you have an image collection stored in BMP or PCX format then you are likely to significantly reduce the disk space requirement by converting the files to JPEG.  However it is important to realize that in compressing files to JPEG some picture degradation occurs.

Whenever you save a JPEG you are asked for a compression setting.  100%  means little compression, i.e. big file, high quality.   1% means maximum compression, i.e. small file, low quality.  We recommend using 75%, at this level you receive little noticeable image degradation, but a significant amount of compression will be obtained so the file size will be very small.