In this case, the issue of resolution only pertains to raster images that are to be printed, nothing else. It is common to think of resolution in terms of DPI, or dots per inch, but as we will see, this is incorrect. Photoshop has a command just to take care of this issue, Image Size. All images that are prepared for print should pass through this dialog box. It is divided into two sections: Pixel Dimensions and Document Size. A quick look at its options shows that resolution is connected to the printed (or document) size and is actually measured in PPI, or pixels per inch. Inches (or centimeters in most of the world) are a real world measurement and only apply to printing on paper. Monitors, mobile devices, and other screens measure increments in pixels, not inches. We often think of high resolution as 300 PPI and low resolution as 72 PPI. This is true, but remember that you only need to worry about it when printing. Since your monitor measures images in pixels and your printer measures in inches, you need to translate pixels to inches when printing. That’s where Resample Image comes in. Resample means to add or remove pixels. No matter what resolution an image has when you open it in Photoshop, you can change it here in Image Size. Uncheck Resample Image to lock the Pixel Dimensions. Only the print size changes; the number of pixels stays the same. Therefore, resolution is relative. Using Photoshop, you can adjust it to what you need. Just make sure you have a large number of pixels to start with.
It is a common misconception that you need to work in 72 PPI on images for web. The truth is that browsers, monitors, and other screen devices don’t use resolution because they don’t see inches. You can work in whatever resolution you want; just keep the pixel size in mind. It is actually a good idea to work larger than your final image because you have more detail to work with. You can even work in high resolution or print size. To make the final web image, the best way is to use Save for Web. With this dialog box, you can choose the format you need, adjust compression (if applicable), and resize the image. Notice that here there is no resolution, only pixel dimensions. No matter what resolution your image has, if you make a JPG, PNG, or GIF with Save for Web, then open that image up, its resolution will default to 72 PPI.
See full post here: Digital Illustration with Adobe Photoshop CS6 Extended2012-12-28.