Adjust the image to fit the size of the space it is in by doing the following:
Place your mouse on any of the four edges of the image.
Start to move your mouse around when your mouse cursor turns into an arrow.
Drag your mouse around to fit the image as you wish.
Check for how large the image is by looking for the width property in the source. Follow the steps below to achieve this:
Click the edit source button in the edit panel above the image.
Copy the width of the image from the edit source tab.
Perform a resize on the image so that it doesn’t look stretched in Outlook versions that ignore the size attribute. Follow the steps below to do this:
Click on the edit image button in the edit panel below the image.
Click on the resize image button in the edit panel above the image.
Paste the width size in the width and height field, check apply and save image.
*Below is a link to a video that shows how to do the resizing.