What are the differences between HTML5 and (pre-5) HTML? How can I make HTML-like forms in Markdown? What is difference between Hypertext and Markup in HTML?
You set the inches and the number of pixels in each of those inches. Photoshop removes or adds data to fit the number of pixels in each of the inches you specified. Note: The original image used in the examples below is 4x4 inches, 100 ppi, 400x400 pixels at 468.8 KB.
When you are creating your logo or image for your signature (in Photoshop, for example), you need to set the resolution to 300 dpi (normally for web applications it would be 72 dpi). You then set the size in inches or cm - for the actual display size - and let the program calculate the pixel size.