Digital images -- particularly high-resolution large ones -- can consume considerable hard disk space. To reduce file size, lossless or lossy compression schemes are applied. As the name implies, lossless is compression without loss of detail. Greater compression is achieved with lossy compression that actually discards some data by an algorithm determining similar colors to be of less importance. This can produce undesirable color compression artifacts such as blurs, color bleeds, and square blotches. At small sizes, these anomalies may not be immediately visible. However, artifacts are accentuated when enlarged or when beginning with a very low resolution digital image.
LZW compressed TIFF is lossless compression, whereas JPEG is lossy. Many images are edited in more than one session. Each time an image opened, edited and saved with lossy compression, it degrades further, thereby increasing the number of artifacts. Though they may not be noticeable at small sizes, artifacts can become quite prominent when enlarged on posters.