with your image resolution? Nothing happens it remains the same till you change it in Image Size dialog. Image > Image Size.. Magnification is for your convenience to see enlarged image nothing really happens to actual resolution of original image.

Go to Image > Image Size. If you want to keep current Width and Height of the image uncheck Resample image then type new reolution in Resolution field. When you want to resample image (to change Width and Height to match new resolution) check Resample Image and type desired resolution in Resolution field.

Resolution of image is used to determine the quality of image in digital camera.

Right click the image, choose properties and go to the summary tab. You will find there the image width and height. Together they give the image resolution.

go to http://www.shrinkpictures.com/ its a online image resizer and you can set the size and image quality. hope this helps