Pixelated
Pixelation occurs when 32 bit images are converted to 8 bit images. 8 bit color-resolution allows only 1024 different colors. Colors, which are slightly different, are mapped to the same color during rendering, leading to the pixelation effect. This effect is not reversible: Once pixelated, the pic will always stay like that and look bad.
Pixelated pics are being rejected by VW.
Artefacts (a.k.a., artifacts) are another fact of digital life. These are patterns created in an image by the application of compression. They will often be very marked at points of discontinuity within the image, such as an abrupt change in brightness. Click the photo below to show it at full size and you will see artefacts in all sections of the image. These were created by heavy JPEG compression, and, once more, they are irreversible.
A few years ago artefacts were a common feature of digital photos, but improvements in internet speeds and storage capacity have reduced the need for the heavy compression which is the major cause of artefacts. It is however still quite common for unskilled users of editing software to produce images with spectacular degrees of artefact distortion. This inadvertent and unnecessary distortion occurs if an image is edited in several different stages, each time saved in JPEG format. Each time a JPEG save is made, more artefacts are created and the effect is therefore cumulative. To avoid this problem, always save an image you are editing in a lossless format such as TIFF at each intermediate stage. Only convert to JPEG during the very final stage, and also you make sure you retain the final lossless TIFF file in case you want to make other changes later. TIFF files are are much bigger than JPEG, but with today's low prices for storage, that should not be an issue.
