ZIP, ARC, RAR, SIT, and other archiving formats are lossless; they are intended to compress data and reconstruct it to its original state. If it were lossy, you wouldn't be able to ZIP computer software, because the code would be messed up. Lossless compression has its limits, because there's only so much redundant or repetitive information you can flag. Nothing really gets thrown out or lost, it just looks for repeating patterns, makes note of where they occur, and keeps only the first instance of that pattern. When the file is unzipped, it's reassembled. Many image file formats support lossless compression, so when you use them, you end up with smaller file sizes, and the image itself is unchanged. Examples include GIF, TIFF, and TGA (Targa). Even Windows BMP has a compression option called run-length-encoding (not very efficient, but better than nothing). The PNG image format is also lossless, has good compression, and supports alpha channel; it works like GIF, but can support 16- and 32-bit color images. I don't believe that regular Poser supports the format, although Pro Pack does. You can convert to and from PNG with third-party software, in any case. JPEG and a few other formats use lossy compression. The compression loses some of the original information. It takes advantage of the fact that small changes in color are less noticeable than changes in brightness. The compression level is variable, so you can increase the compression to get smaller files; the tradeoff is increasingly worse image quality. PSD (Photoshop's native format) is great if you want to maintain all of an image project's original layers or channels. Its compression scheme is run-length-encoding (lossless), so it's perfectly safe; no loss of image quality. I prefer to use lossless schemes to archive my images, despite the fact that the compression isn't as efficient, because (like George Lucas) I might come back and tinker with them again. I need to work from the originals, because saving and resaving to a lossy format results in cumulative image degradation. When I post an image to the web, I'll crunch it down to something manageable with lossy JPEG compression, because a lot of people are stuck with slow dial-up. I still have the original, so it doesn't matter.