There are 2 ways to resize an image, depending on what type of image it is.
1. Resizing an Image Card
You can resize an image card by clicking on the image and then using the slider on the left hand side:
2. Resizing an Inline Image
If your image is part of a Text Card then you can resize the image directly on the page. First, click the image to activate it. When the image is activated it will have a small square in the bottom right hand corner:
You can click and drag this square to resize the image: