Sips - macOs builtin scriptable image processor
MacOS has a builtin tool sips that allow you lookup or edit raster image files and ColorSync IIC profiles. This is what I use for resizing images.
$ sips --resampleHeight 1080 *.jpg #changes all jpg files to have 1080 pixel height
Composite - member program of ImageMagick for overlaying images over one another
ImageMagick's composite tool can be used for watermarking your images. You'll need to create a watermark image to overlay onto your main image. First, install ImageMagick . The composite tool comes bundles with the installation.
$ composite -dissolve 50% -gravity southwest <watermark-file.jpg> <target-file.jpg> <output.file.jpg>
Reducing the file size of your image facilitates web page loading times and slows the filling up of your server or VPS provider (meaning higher costs). In addition, watermarking provides a way for you to control how your images are shared and gives credit to you, the creator.
Images on this site are all downsized. Images may or may not be watermarked. If you're interested in a full-resolution image, contact me at the link above with a description of your use case. Generally, I would only ask for payment if it's a business use case.