techniques for manual ordering of data ?

I tried googling, but the keywords here are so general, that I couldn't find
much useful.

I am looking for reommendations on how to build an ordered list of media (in
this case photographs) in a particular "context". What I have - which I have
somewhat simplified maually - is:

CREATE TABLE contexts (
photo_id int8 REFERENCES photos ...,
catalogue_id int8 REFERENCES catalogues ...,
collection_id int8 REFERENCES collections ...,

ordering int
);

Here, a photograph may appear in multiple catalogue/collection places. What
I want to do is to allow the admin to reorder the display by using + / -
style buttons. My initial thought was to use an arbitrary "ordering" column,
like old BASIC line numbers, which would be initially allocated in 10s or
100s, and then updating the value via the UI and occassionally applying a
"renum" style function when the list gets uneven.

To avoid performance hits through too many updates, I am trying to avoid the
admin UI from UPDATEing all the rows in a specific catalogue/collection
every time a button is pressed - both the SQL and the subsequent fragmented
table. Hmm.

Anyone got any "prior art" or suggestions on how they acheive this ?

Note that I am NOT including ordering based on an photograph specific
metadata like time or location. That's another story and another interface.

rgds,
--
Peter