File types: 

Image have to be saved as files, there are many formats to choose from:
Of all the different file types the most useful are:


Photoshop .psd:
Advantages:
useful when saving work in progress, there is no loss of image quality and images can be saved with layers.
Disadvantages:
very few other programmes will understand open or import psd files.

Gimp .xcf:
Advantages:
useful when saving work in progress, there is no loss of image quality and images can be saved with layers in Gimp.
Disadvantages:
very few other programmes will understand open or import xcf files.

Tiff .tif:
Advantages:
providing you don't use the compression mode no loss of quality, accepted by most D.T.P. programmes
Disadvantages:
uncompressed the files are large, does not support layers
New versions of Photoshop does - but don't do it!

Jpeg .jpg:
Advantages:
variable compression allows file sizes to be made much smaller for saving to disk and for sending over the inter-net or by e-mail (files can be one tenth of the size of a tif or psd). Standard image format for pictures on the web. Accepted by a large range of other programmes. Standard format for Photographic lab printing.
Disadvantages:
compression loses some quality especially if the file is opened and saved several times, does not support layers.

Compression with jpg files
Saving at high quality low compression will create a file without significant loss of quality, using the wrong settings will degrade the image.
The loss in quality is repeated each time a file is saved as a jpg.

 

Image saved as a high quality / low compression jpg file. 		 

Image saved as a low quality / high compression jpg file.
 
File sizes:
A rough guide to file sizes.
A 10"x8" image at 300dpi will be approx 20mb
and a 16"x12" image will be approx 50mb
A 4 megapixel camera will give an image of 11mb (saved as .jpg - approx 2mb)
A 6 megapixel camera will give an image of 17mb (saved as .jpg - approx 3-4mb)