To begin, open the photo in Adobe Photoshop by selecting the File menu the option Open. Then open the Image menu and select Image Size. A window opens where you can specify the new size of pictures in pixels.
Set the desired width and height of the image. In paragraph Pixel Dimensions you install a direct photo resolution, and Document Size you can determine the size of the printing sheet with the image in inches.
In the field Height and Width enter their values. They will automatically adjust to the program under the photo's aspect ratio, so it is not deformed and stretched, if you have a tick in the Constrain Proportions item. Click OK and your photo is reduced.
You can also change the extension of the pictures using its automatic optimization for your web pages without loss of quality. To do this, open the File menu and select Save for Web & Devices. As a preset, select JPEG High. The same can be done by selecting the menu File -> Save As a JPEG High Quality 8.
If you don't want to lose the quality of a raster image, never enlarge a photo from a smaller resolution, otherwise the quality will suffer considerably.
To get a picture of appropriate quality is possible only if you reduce the resolution, changing the larger photo reduced. If you have a small photograph, to increase its resolution without pixelation will not work.