Stairstepping can be introduced when a picture is made smaller or larger without being resampled at the new resolution. See pic below. Straight diagonal lines don't appear straight but more like discrete horizontal and vertical bars (like stairs...steps).
No need for Photoshop. Irfanview is a nice free imaging software that allows to easily resize/resample among other things
When I download a picture that is bigger than my screen resolution, I usually use Irfanview to cut any unwanted part and then I resample the remaining image for my screen resolution.