Do not know Picasa2 (apart from what I Googled
here). Personally, I never trust programmes like these (better to use Martin Wright's
TView) to view MSFS textures.
Anyway, what
I would do in such a situation is: backup (copy) your entire FS Aircraft folder (preferably to some removable media: CD, DVD) and do what the programme asks. If it does delete the textures from the a/c folders ... well you have a backup to replace them with (and you have learnt a valuable lesson).
Good Luck.