I think the imgproc shortcode in the docs is very helpful.
But, to add to @ju52’s sample, here is another.
Let’s say you have a blog post some-post, and your image is part of a leaf bundle:
├── content
│ └── blog
│ └── some-post
│ ├── index.md
│ └── space.jpg
You could define a shortcode, layouts/shortcodes/img-resize.html:
{{ $original := .Page.Resources.GetMatch (printf "*%s*" (.Get "img")) }}
{{ $new := $original.Resize (.Get "size") }}
<a href="{{ $original.Permalink }}"><img src="{{ $new.Permalink }}"></a>
Then use it in your post like:
{{< img-resize img="space.jpg" size="500x300" >}}
This simple shortcode resizes an image, then gives a link to the original.