GIMP is an image editing software very similar to Photoshop. You can install it on windows, linux or mac - it's open source.
We will look a bit at the interface and will resize an image. After you install Gimp go to File > Open and open one of your image. The Gimp screen will look similar with the image below:
In the left side you have the tools with the options of the selected tool and in right side you have the Dialogs with the Layer Dialog open.
Look in top bar to see how big is the image, most of the time the image is too big to upload it on the internet or to send it by email. So go to Image > Scale Image... and change the with and height to more reasonable numbers.
Where do I go from here?
Explore. Gimp does not come with a help installed but you can go at http://docs.gimp.org/2.8/en/gimp-first-steps.html and go on.
Note: For stability on Windows 7 disable window transparency. Right click on desktop and go Personalize, click on selected theme , on the bottom Window Colour and here desable Windoe transparency.