By default, and like it or hate it, the macOS keystrokes to capture full app screen (⇧- ⌘- 5) will add a subtle drop shadow around your saved image.  This can make it more visually appealing when adding a screen shot or image to a document.  The convert app, part of the ImageMagick distribution, has a nice command line function to add this same drop shadow effect to any of your images (e.g., a partial screen capture using .⇧- ⌘- 4)

Once installed, you can use the following command line to update your image with a drop-shadow:

convert "Screen Shot 2021-07-07 at 09.47.13.png" \( +clone -background black -shadow 80x20+0+15 \) +swap -background transparent -layers merge +repage "Screen Shot 2021-07-07 at 09.47.13.png"

which will turn this:

into this:

Pro-tip: You can create a pseudo-function in your BASH or ZSH environment - e.g., in your ~/.bashrc file, add this line:

shadow () { convert "$@" \( +clone -background black -shadow 80x20+0+15 \) +swap -background transparent -layers merge +repage "$@"; }

and then source your ~/.bashrc file (first time) to activate it:

source ~/.bashrc

Now, from your command line, you can add a drop shadow to any image file by simply typing:

shadow "Screen Shot 2021-07-07 at 09.47.13.png"

(remember to add double-quotes around your file if it contains spaces)

Done.