You can follow the instructions here to get to the files area and upload a file.

You can place it anywhere in the files system - but a custom folder may work best. If it's a link you plan on using for your public site, make sure to keep it in the public files area.

After doing this, open up the piece of content where you want to place the image.  In the body text area, you'll see the image embed button (it looks like a little picture of mountains and a sun). Click it.

This will open a dialogue box with a url field for the image you want to use. Click the "browse server" to locate the image you uploaded to the files area. Double click it. This will put the image url into the url field.

You can sue the image property options to adjust the image. When you're done, click save.

Protips:

• Use a percentage for the width of the image, and leave the height blank to make the image responsive. This will allow the image to resize of different screen sizes.
• Hyperlink the image to another page by highlighting the image after embedding it, and clicking the hyperlink button (looks like a world with chainlinks).