First, lets take a look on what problem will this tutorial solve, so you can decide if you want to read more :)
We will write 2 functions for creating thumbnails from random image (png, jpg, gif). First function will take thumbnail width and height set by you, and cut the “most important” part from the image (I’m assuming, that you are creating thumbnails of websites, so center top is the area I’m talking about) according to thumbnail ratio, and resize it into your thumbnail in exact size that you’ve set. Second function will take max thumbnail width and height set by you, and resize the image so it will fit your max values, but with preserving its original resolution ratio.