Nick Barker's notes on setting page properties when building a web graphic from scratch using Photoshop.
Setting resolution, colour mode and bit depth:
To open a new page: file > new. You get a pop-up menu with choices:
Dimensions
When you set the dimensions for your image (use pixels) it is a good idea to work at the maximum size you are ever likely to need for the project. Consider possible future requests for the artwork or posters, prints, etc.
You can then reduce the file size for the web but you cannot increase the file size at a later date and retain the quality for print.
(This is not necessary if you know it will not be wanted for another project.)
Resolution
The resolution is irrelevant for a web graphic but you might want to set it for possible future printing.
Colour mode
The colour mode should be set to RGB 8-bit even if your image is going to look black and white. Web browsers cannot read CMYK images.
Bit depth
Bit depth is the amount of colour information in each pixel and only 8-bit can be displayed in current web browsers.
Background
Choose a background. It does not matter what you choose as the background can be changed later. It is easy to work with a white background.
Advanced
Make sure the colour profile is set to sRGB, which is for a typical PC monitor.
Click OK
Save your file in the psd format to keep all the layers separate and editable.