Save one of these images by right-clicking it and selecting 'Save Image As' from the popup menu. Give it a useful name and remember where you saved it. If you are viewing this page from within the Second Life web browser, press the 'Open in My Web Browser' button to open the page in an external browser. You'll otherwise be unable to right-click the images.
Then from within Second Life, select Upload Image from the File menu. Locate your image, select it and press the 'Open' button. Preview the image to make sure you've selected the right one, and then press the Upload button. It will cost L$10 to upload an image.
The larger image will look better in-world but will take longer to download and will use more graphics memory.
To apply an image to an object, find it in your Second Life inventory and drag it to the surface you want to apply it to. To apply it to the Online Status Monitor, drag the image over top of that name at the top of the panel.