Personally.. I use Flash Drives myself..
But using a SD Card or a Flash Drive you store your images the same way.

Those images have to be in your download folder, and you simply copy them one at a time from there and paste them into whatever you have plugged into your Chromebook.

When you put your mouse pointer onto something in your download folder you right click your mouse and chose copy.

Then you click onto SD Card or Flash Drive and go there and paste it in.