How can I highlight text on a screenshot using Markup in macOS?
You can open the image with Preview and build your own sticky. Under the Tools menu go to Annotate and select a rectangle. Adjust size of rectangle and then set fill color to yellow and the line color to none. This results in a yellow rectangle. Now under Annotate select text. This puts a small text box on your image. Move this to inside yellow box and type in your own text. Now save the file as a  jpeg or whatever you like and your image will have a yellow sticky on it.  This maybe be to long a process for you but it works. See image below for an example.