There are lots of programs that enable you to modify and reduce an image to a specific size.
There are lots of easy-to-use programs that allow you to modify or reduce an image to a specific size: examples include Adobe Photoshop, Gimp, ...
Another quick way of adapting an image to a specific size is to use Cropp me.
But if you want to touch up, modify and reduce an image in a more professional way, there here is a list of programs you can use:
Desktop editors · Software
Online Editors · Web