big surgraphicsimage editing
I thought i'd be able to do this with the built-in Preview app, but I can't find a way to do that.
I have an image i'd like to reduce the opacity to 50%.
Can anyone advise if there's a way to do this with built-in applications that come with MacOS or if this needs GIMP or another image editing tool? I'd rather not install GIMP (if I can avoid it) just for this.
You said "free," but you also mentioned "nothing expensive like Photoshop"; there are solutions to your problem that do cost something but are nowhere nearly as expensive as Photoshop.
Pixelmator is great for certain kinds of quick image editing. It's only $30 right now, though that will probably go up when the next major release finally comes out; I believe it's normally $60.
There's also Acorn, which is very similar and perhaps a hair's breadth easier to use, though since they've already had their first post-App Store major version release, it's more expensive at the moment ($50).
If it absolutely must be freeware, do have a look at Seashore, mentioned in a comment by Ian C.
Preview supports AppleScript which allows you to open and save files from CLI. There are commands for open and save.
For example, to open a file with Preview:
tell application "Preview"
open "/path/to/file.png"
end tell
You can run AppleScript as a script file, or from the command line using osascript -e "…".
osascript -e "tell application \"Preview\"" -e "open \"/path/to/file.png\"" -e "end tell"
You can find the full Preview AppleScript dictionary by opening Script Editor.app (from the Utilities folder), choosing File → Open Dictionary and selecting Preview. This will show you many of the commands that Preview supports with the syntax for how to use them.
Best Answer
Yes, you're going to need a graphic image editor, be it GIMP, or Pixelmator, or Affinity Photo, or Adobe Photoshop, or other.
Having some sort of program of this type is bound to come in useful again. Preview has its uses, but it's not a general-purpose image editor.