These few lines gives a quick control on the image size in case you don't want a full width or full screen image. You create the MD image as below to resize the image by 50% for instance:

This will create the below img tag in HTML. The @50% will be truncated from the caption, but it will be still present in the ALT. You can use whatever size after the @ between 1 and 100. Then the integer has to be followed by %.
<figure class="image">
<img src="https://blog.yourdomain.com/storage/image.png"
alt="This is the ALT and caption of an image.@50%"
style="width: 55%; display: block; margin: auto;">
<figcaption>This is the ALT and caption of an image.</figcaption>
</figure>
Just paste the code snippet to your Ghost Blog admin Code injection's Site Footer and you are good to go. The base caption function made by Kevin Chung here.