A principle with graphics file formats. Converting JPEGs to a lossless format like GIF or PNG is usually pretty braindead--the quality remains the same, and the compression ratio is even worse than if it weren't JPEG to begin with, because the JPEG compression algorithm introduces artifacts into the image.

Example: A photo I took of Chen Shui-bian is 1,878K as BMP. Converted to GIF89a it becomes 235K. Converted to a high-quality JPEG it becomes 134K. Converted back to GIF89a it becomes 266K. That's a hefty 31K of pure JPEG artifacts. Iterating this process is likely to fog up the image and cause the image size to balloon.