Scanning an image to produce a digitised version reproduces the image, and thus requires permission (unless the copyright has expired). You will also generally need permission to produce a new image by altering the digitised image, if an important part of the first image is recognisable in the new image.
You should also ensure that any changes you make do not result in the work being treated in a way that is prejudicial to the honour or reputation of the artist, or you may be liable for infringing their moral right of integrity.