Vector and Raster Images

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Resolution

DPI (Dots Per Inch)

Only raster images are made of dots, so only raster images have a resolution, or a number of dots per inch (or sometimes, "PPI" which stands for Pixels Per Inch). The DPI of an image is defined when the image file is first created. While graphics applications like Photoshop can enlarge a raster image after it is created, the enlarging process requires the application to guess at the color of the new pixels needed to fill the space between the original pixels.

More dots per inch = higher resolution = BETTER.
Vector

Compression

Because of all the dots, high-resolution raster images can get quite large. Large files take longer to transmit over the internet and take more hard drive space to store. Compression techniques try to reduce the file size while maintaining reasonable visual quality.* The example on the right, above is heavily compressed using the JPG compression techinique, as a result that image is useless for printing.

Fewer dots per inch + low resolution + compression =
MUCH WORSE

Tip

The size of a file can be a good indicator of resolution, bigger files are usually higher resolution and therefore can be of higher quality. If you're trying to find a high-quality image file, start with the biggest files first.

*There are two types of image compression, lossy and lossless. Lossy compression, like that used for .JPG files is destructive, it actually throws out or "loses" data to make the file smaller. The amount of destruction can be adjusted when the file is first saved to the .JPG format, however, once the file is compressed and saved, the data thrown away to make the file smaller is lost forever. Lossless compression, like that used in .ZIP files, simply tries to pack the data as close as possible without throwing anything away. Files saved with .ZIP compression can be fully restored to the original quality.
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