Understand the difference between Raster (pixels) vs Vector graphics.
http://www.youthedesigner.com/2012/08/12/how-to-explain-raster-vs-vector-to-your-clients/
http://thecreativestack.wordpress.com/2012/09/26/raster-vs-vector/
http://youtu.be/fy9Pby0Gzsc
Understanding Image Resolution and Pixels
http://www.photoshopessentials.com/essentials/image-resolution/
Understanding Resize vs. Resample
http://tv.adobe.com/watch/learn-photoshop-cs6/understanding-resize-vs-resample/
http://www.photoshopessentials.com/essentials/resizing-vs-resampling/
Understanding RGB, CMYK and Color Channels
http://www.photoshopessentials.com/essentials/rgb/
Adjusting Images
http://www.photoshopessentials.com/photo-editing/adjustment-layers/
http://www.photoshopessentials.com/photo-editing/levels-curves/
Embedding your Contact And Copyright
http://www.photoshopessentials.com/essentials/file-info/
Saving for Web
http://www.photoshopessentials.com/essentials/save-for-web-cs5/
Saving and File Formats