Photography

I take a gazillion photos, and am constantly resizing.
I use two apps on the mac: SuperPhotoUpscaler or BatchPhotoResizer (cuz they'll do several at a time)
Preview app (or any viewer on any computer) should offer you a "save as" usually with resizing options,
but the easiest for mac folks is to email themselves (or others) a photo, and the Mail app will ask you if you want to resize it, full, large, medium or small. It ain't elegant, but it's easy.

If you can't right-click (control click on mac) to save an image, you can almost always just take a screenshot.
Built-in screenshot capability or apps have a way to save just an area you select, not just the entire screen.

Of course, I sometimes "edit" photos to make a fun meme, but that's another story altogether.