When adjusting your Custom Email options, you will need to know the recommended image sizes. Here is a guide to help:
Masthead Logo (for both Queue and Welcome emails): 575 x 150 pixels
Share All button: 300 x 50 pixels
Share button: 131 x 30 pixels
Email button: 120x 30 pixels
Web Interface logo: 456 x 194 pixels
For best representation in Outlook, it's recommended that you save your images at 96 dpi resolution.
Once you have the correct size images on hand, you can find more information on how to customize your template here.