There are two ways to change the size of a picture: changing the actual physical size of the photo, and changing the resolution of the photo to change the size of the picture file. No matter how the image is changed, the quality is affected as well.
Changing the size of the photo requires a change in the size settings for photo printout. Most graphics programs allow resizing simply by dragging an edge of the photo in the display area. When settings are chosen to allow for changes without altering perspective, dragging a horizontal edge also changes the vertical size of the photo at the same rate. This allows the user to enlarge or shrink the photo without distortions due to a photo suddenly becoming too wide or too high.
Without the lock-on perspective, the photo is resized on one plane only, either horizontally or vertically. Making photos larger tends to blur the image.
The second method of resizing a photo involves changing the photo's resolution. This involves a change in the pixels per inch of the photo, which is the pixel density of the picture. By lowering the pixel density of the photo, the file becomes smaller, which decreases storage space needed to save it. Lowering the density can also cause blurriness to occur, as fine details are lost during the process.