Collect and Image
The easiest way to save an image to your library is to click on the Diigo icon that appears in the corner of the image when you move your cursor over it. For whatever reason this icon may not always appear, or you may find it annoying and disable it, in which case you can save an image to your library by right-clicking the image choosing "Diigo..." then "Save Image to Diigo". Either way you choose to save an image, there are two save options: attach to URL, or tag as standalone. If you attach an image to its URL, the page it is on will be bookmarked in your library, and the image will be one item in that bookmark. If you save an image as a standalone, it will appear simply as an image in your library.
Capture and Mark-up an Image
Choose "Screenshot" from the Diigo extension menu. Next, click and drag to select an area of the page to save as an image in your library. When you have selected an area of the page, the capture toolbar will appear at the bottom of the selected area. Choose text or a shape, choose a color, then click-and-drag to draw a shape or click and type some text. You can click the curved arrow on the toolbar to undo your last action. When you complete however much mark-up you wanted to do (you don't have to do any), just click the floppy disk icon to save to your library.