A brief outline
Clive R. Haynes FRPS
To reduce the storage size of an image, use a compression program and for preference, one that is 'non-lossy'. If you don't have any compression software with the image package your using then WinZip or Drive Space are useful types. Saving as a compressed file will often loose not only definition but also other features such as 'Layers' or 'Channels'.
This is a widely used compression but it is 'lossy' - that means information is lost in the compression process. The image may look fine on the screen but as a high resolution print output losses will result in degraded quality.
JPEG compression offers different levels of compression - choose the most suitable.
If one successively 'opens' and 'saves' a JPEG then the losses accumulate to become ever greater. Also if one makes further compressions to a JPEG file the degradation escalates.
Therefore choose the JPEG compression only when the image is complete.
JPEG is fine for web images and sending via email.
This compression is non-lossy, however unfortunately some programs/applications will not recognise it.
TIFF is a cross-platform file that typically can be read by both PC and MAC users and LZW is a TIFF compression option.
This is an 8-bit file format, this means that it can accommodate 256 colours. It's usually used for publishing graphics and small 'thumbnail' images. However before using, the image must be adjusted to 'Indexed color'. It's fine for web use, as most monitors will happily display a maximum of 256 colours. Whether you use GIF or JPEG depends upon your need and the web browser you are using.
PICT (MAC only)
This compression is particularly good for archiving images with areas of contiguous colours - such as subjects against plain colour backdrops. In addition the PICT compression can be additionally compressed by JPEG (should you so desire).
Not so much a compression but one that you may come across!
This stands for Encapsulated Post Script and are the preferred format for page layout documents. The DTP package Quark Xpress uses a version of EPS known as DCS (Desktop Color Separations)