A pixel is the smallest piece of data in an image, which may comprise thousands or even millions of pixels. 1 megapixel is 1,000,000 pixels. For example, if picture dimensions are 2,592 by 1,944 pixels then it contains 5,816,448 (2,992 x 1,944) pixels, or about 5.8 megapixels. Adobe Photoshop allows a user to manipulate the pixel image size in a simple way.
Launch Adobe Photoshop software.
Click "File", then "Open." Browse your computer and find an image file. Double-click on the file to open it.
Click the menu "Image", then "Image Size." Check the box "Constrain Proportion" in the bottom of the image size window. Note that If this box is unchecked then picture proportions would be distorted.
Multiply picture width by the height (in pixels), given in the field "Pixel Dimensions", and then divide the product by 1,000,000 to calculate current megapixels.
Enter a larger value in the field "width" to increase picture megapixels. Note that the value in the field "height" will automatically increase to maintain picture proportions.
Calculate the new picture megapixels as in Step 4. Click "OK."
Click the menu "File" > "Save As." Type a filename and click "Save" to store the image with the new megapixel count.