If you have either heavily cropped an image, and/or you are intending to create super-large prints from it, then upscaling can help you maintain sufficient resolution.
To do this from the Photoshop interface head up to the Application Bar and ‘Image > Image Size’.
There are a couple of ways to do the upscaling, you can either select the size you want using the ‘Fit To’ pull-down menu and then use one of the print sizes available. Or, you can go ahead and adjust the size directly in the Width & Height boxes, e.g., if you want to double the resolution, just go ahead and double the width and height.
You can let Photoshop automatically select an upscale algorithm, or you can manually select one from the Resample pull-down menu. The Preserve Details 2.0 model is the latest from Photoshop and typically gives the sharpest results.
Just bear in mind when doing this, that Photoshop is having to create those extra pixels using artificial intelligence. So the more you up-scale, the higher the chance you have of introducing unwanted artifacts into your image. Once you have upscaled, it is a good idea to take a close look at your image at say 200% magnification to check that you are satisfied with what Photoshop has done.
