GIF can have transparent backgrounds. Animated GIF's have the GIF extension also, and the simple ones are not that large. JPG - Use for photos. Choose 16 million colors. No transparency available.
Photos can be huge in size.
JPG will compress them significantly. To capture an image off the net, hover over the image and right click to copy it to clipboard. Paste it to a directory where you can access it. Only a very limited amount of basic information was presented here.