UNDERSTANDING RESOLUTION AND PIXEL SIZE

Web browsers ignore any resolution information recorded in an image. (The exception is Scalable Vector Graphics, which requires a plug-in for the web browser and cannot be created in Photoshop or ImageReady.) There's a reason for this: Your monitor isn't capable of showing 300 dots per inch, or 1200, or whatever resolution you may have set for an image. Images are displayed onscreen strictly according to their pixel dimensions, not their print dimensions. Consider resolution to be an instruction to the printer about how to reproduce the image on paper, not an instruction to the browser about how to reproduce the image onscreen.