[Iñigo]

If I've understood correctly, it should not be too hard for .pdf files.

Locate the images files in filesystem used in boot and shutdown processes.
Change the shutdown process with a script that 1. change those images with a
screenshot of the page currently reading, and 2. do a shutdown

Page number could be extracted from the manifest.

This is a sample python code that creates an image of a pdf file page.
It shouldn't be difficult to translate to C or lua.

import gtk
import gtk.gdk
import poppler

def build_cover_pdf_poppler(f):
    """poppler method. Needs X"""
    dst = mkstemp()[1]
    try:
        doc = poppler.document_new_from_file('file://' + f, None)
    except:
        os.unlink(dst)
        return
    pag = doc.get_page(0)
    w, h = map(int, pag.get_size())
    pb = gtk.gdk.Pixbuf(gtk.gdk.COLORSPACE_RGB, False, 8, w, h)
    pag.render_to_pixbuf(0, 0, w, h, 1, 0, pb)
    pb_scaled = pb.scale_simple(TN_W, TN_H, gtk.gdk.INTERP_BILINEAR)
    pb_scaled.save(dst, 'jpeg', {'quality': TN_QUALITY})

Iñigo