1. Click on the rectangular marquee tool (the tool in the upper left-hand corner of the tool box) and drag to select the part of the picture you want to keep.

2. Set the Feather option to 0 in the Marquee Options box.

3. Choose Image > Crop. This crops the photograph while maintaining the same resolution.

Note: Don't overcrop your photographs! Here is an example of cropping (original photo is on the right):