Width and height of images
When an image is added to emails it will be automatically resized to fit the container it's added to. This resizing takes into consideration padding around the images. For example if your container is 660px wide and you apply a 30px padding on the left and right sides, the image in this container will be displayed as 600px wide. This works well in all cases of desktop and web based email clients.
Images on mobile devices
The "Show full width on mobile devices" checkbox on the image setting window controls whether images are automatically scaled to fit the entire width of small screens.
Showing images full width on mobile phone is best practice with the exception of small images such as social media icons or logos that shouldn't be scaled up.
For such small images turn off the "Show full width on mobile devices" option and at the same time be sure that the image in question is not too wide. For example if you have a logo that you don't want to scale up on mobile screens you must ensure that the image file is no wider than 300px. Click the "reset" icon to find out how wide the image file is. If it's wider than 300px then you should go back to the image library and select the "Choose resized" option where you can resize the image.
Centre aligning images
To centre align an image click next to the image and click the centre align icon in the toolbar.
Animated GIFs or other animations
You can add animated gifs to emails the same way you would add non-animated images. These animated gifs will work in most email clients. Of the ones that don't support it the first frame will be shown and the image will remain static.
Related article: Videos in emails.