Below you will find some useful links and reference images.

On Leuku's Finnish language pages there is a wealth of information in PDF for teaching Photoshop and covering topics like colour management and file formats and Photoshop keyboard shortcuts.

Grays on White  and Grays on Black  
JPG -files

Open these two pictures in Photoshop and get a surprise! The grey squares in both pictures are identical but look entirely different because of way our brains perceive the background.

The percent numbers are from Gray Gamma 2.2 and they are completely different when viewed in different Gray 'Working Spaces'.

Testimage.jpg -file (use Save Target as...)

This Adobe RGB file contains a wide range of virtually unprintable colours demonstrating the limitations of 'Out of Gamut' colours. Use the View- Proof Colours to get some idea of just how much colour is changed.

http://www.planetphotoshop.com and http://www.planetphotoshop.com/resources.html

The above links are a gateway to a world of Photoshop resources and discussion forums.

http://www.fredmiranda.com/

Software, Forums, Reviews, Articles and just about everything else on Fred Miranda's site for the digital photographer.

www.adobe.com -link

Adobe's pages provide a wide variety of information and resources including support and downloads.