Depends. Best case, right click the image and choose Copy. Open Photoshop, and use Ctrl (Cmd) n to open a new document the size of the image in the clipboard, and Ctrl (Cmd) v to paste.
Some images do not let you copy them. In that case set your browser zoom to 100%, and use Alt Prt Scr to grab the entire screen. (I don't know how Macs to a screen grab).