On a Mac press Cmd-Shift-4 then draw a rectangle around what you want to capture and release the mouse.
Instead of going to your clipboard, the screenshot is saved to your desktop with the date and time appended.
As Peckris says, acknowledge the source out of politeness. I'm sure that most sites realise that their images can and will be downloaded.
And for those that can't, I screenshot the webpage, paste it into a Word document and then save the image as a jpeg file where it can be cropped with something like Adobe Photoshop.
Spink's Coins of England 2024 has finally included my proposed penny additions of 1847 Medusa, 1858 Large Rose (although they have mysteriously duplicated the entry), 1862 over 1, 1862 VIGTORIA and 1863 open 3.