these can be replaced vía file>import but do remember that the resolution of the picture is very important. A Low-resolution graphic at about 72 ppi (píxeles 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 todo 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 youll have todo make do with that, but youll not get the best result.