Where the DCT image transform is used, for example, they are often 8x8 pixel squares, containing a stripe or "checkerboard" pattern.
Where predictive coding of motion pictures is used, as in MPEG-1, compression artifacts tend to remain on several generation of decompressed frames, leading to a "painting" effect being seen, as if the picture was being painted by an unseen artist's paint-brush.
Where motion prediction is used, as in MPEG-2 or MPEG-4, compression artifacts tend to move with the optic flow of the image, leading to a peculiar effect, part way between a painting effect and "grime" that moves with objects in the scene.
Errors in the bit-stream can lead to errors similar to large quantization errors, or can disrupt the parsing of the data stream entirely for a short time, leading to "break-up" of the picture. Where gross errors have occurred in the bit-stream, it is not unknown for decoders to continue to apply 'painting' updates to the damaged picture, creating "ghost image" effects.
To stop the build-up of compression artifacts, most compression systems occasionally send an entire compressed frame without prediction or differencing.
In MPEG picture coding, these are known as "I-frames".Compression artifacts in picture coding