It is often best to resize images before you upload them to Moodle. This makes their file size smaller, and means less data your students have to download every time they view the image.
Here is a quick instructional video on resizing an image using a free browser-based tool called Pixlr Editor. You don't have to install something, just go to the Pixlr website and start using it!
It is now ready to upload to Moodle!
» Guide to Moodle