Thanks to all for the informative feedback.
as far as I can ascertain then(here I go with my outloud thnking thing again

). jpegs are one of the universal formats for a digital image but DDS is a (compressed?) but still highly accurate and indeed desirable(for Celestians...see issues in this thread..0) & Celestia ( being the uniquely beautiful thing it is)...is able to read both...
I cannot commit the time at this point to converting these files for my own needs but the knowledge is very much appreciated..
Thanking you all
Osmium
Under the bonnet..
Win XP home ed SP2
512 MB RAM
80.0 GB HD
RADEON 9700 PRO display Adapter.
AMD Athlon 2600+
Celestia 1.4.1