Vector vs. Raster
Vector formats use mathmatical formulas to increase and decrease the size of the graphic. Because of this, the edges are very smooth - no matter what size you make the graphic.
Raster formats use pixels - think Roman mosaics - to make pictures. Resolution - or PPI (Pixels Per Inch) readily affects the quality of the image. Think counted-cross stitch patterns, 22 count looks much finer than 14 count.
This is why changing the size of a raster graphic can change its appearance.
JPG vs GIF
JPG - pronounced J-Peg
GIF - pronounced GIFF - with a hard G (or I've seen it JIF, also)
As a general rule:
JPG is used for photos and greyscale pictures with lots of graduation in color. The compression abilities of JPG are superior when there are many colors.
GIF is used for graphics with solid colors or line drawings. GIF's can be made transparent, animated, interlaced.
What is PNG?
PNG - Pronounced PING - Stands for Portable Network Graphics
Uses a "lossless" compression scheme. Can compress smaller than GIF's and have more color.
Interlaced & gamma information (brightness information included)
Can be resized within the web page without loss of resolution quality.
Not ready for prime time yet!
What is GIF89a?
Most monitors are currently capable of exhibiting 72 PPI (Pixels per Inch). Save the Bandwidth - don't save JPG's in high resolution for exhibit on your web site. If scanning - 144 is the MOST you should ever need.
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