What Mark has given you is a great
Global way of setting the default STILL length.
What I might offer is less of a global action. I read your "
For instance, I have a picture I'd like to last 3 seconds" - that knida implies that you already have it IN te project. Use Edit Detail View [EDV] - you'll see it there under VIEW.
1/- CLICK on the STILL
2/- Open EDV and chose "
Selected Events" - you should now see THAT event/Still for changing
3/- Type directly onto "
Length"
DONE!
Now, and here's a real neat tip!!
Select several Events already on the Timeline. Now using EDV HIGHLIGHT all the LENGTHS and yes . . you got it! Just type in directly the length you want, press ENTER and all the Events change too!
DONE!
OK, 'cos you HAVE changed their lengths you WILL have gaps. But I BET there is a "Remove GAPS Script"out there?!??
And that's how I would change the length of ONE or several OR Selection of stills.
Another way is to do it VIA Trimmer. In Trimmer you can TYPE the length for a loop region and then either DRAG and plop OR use FROM Cursor Command.
I like the EDV 'cos you can "pick-off" the stills you want to change on block.
Hope that helps.
Grazie
Clip duration 
Linear Mode
