These can be replaceed via "File>Import" but do remember that the resolution of the picture is very important. A low-resolution graphic at about 72 ppi (pixels per inch) will NOT give a very sharp finish at its original size.
If you grab such an image from a web page (which is generally 72ppi) then you cannot scale the image UP without making the resolution go DOWN and so making the image more 'blocky'. For instance, if you make a 72 ppi image twice as big then the resolution drops to 36ppi which makes the image look really bad! Image files such as photographs are best presented at 150ppi and images with a lot of lineworque such as text are best presented at 300ppi.
Of course, if a web-grab image is all you have, then you'll have to make do with that, but you'll not get the best result.