Suggestions for preview images

The following suggestions have been taken from OFFICE-2306:

Why is there a folder thumbnails if there can be
only one thumbnail? Would it make sense to allow multiple sizes, in
order for the whole spectrum of devices (from small screen devices to
high end systems) to have a pleasing rendering? Question: Are there no
constraints or recommendations with regards to the PNG itself, e.g.
allow/disallow alpha layering/color spaces? Question: What about
accessibility - is there information we can add for blind people that
use a file browser to navigate through a large amount of ODF files?
Where can we store this information, and are there recommendations to
make to those making file browsers (including office applications, who
may include this)? Question: Should we consider allowing an SVG preview
image, if necessary with a PNG embedded? Question: Is there information
we should store in the metadata of the PNG? Question: Some platforms
may require other preview formats. E.g. NeoOffice currently seems to
add a small pdf of the first page for the preview on Mac OS X. In fact,
one could say that a low resolution version of a PDF rendering of an
ODF file would be an interesting addition to the preview concept in ODF
in some cases - it is a frozen, final format version. It might be
interesting to recommend applications should be allowed to include the
possiblity to include full PDF's in ODF as preview. That way hybrid
PDF's are available both ways: PDF contained within ODF ('as last seen
by the author', ODF contained within PDF ('meant as a final
publication, but you can edit it if you want'. Of course these two
mechanisms should not loop.