*I'm doing this from work, I work on a PC, so this won't be exact, but you'll be able to figure out the gaps and name mistakes*
Right Click the Desktop.
From the popup menu, highlight "Automator" (it will slide out)
Click "Make New Automator Action"
Automator will open.
Delete the action on the right, it's the only one.
Click "Finder" on the far left, to see all of Finder's actions.
You need to find "Get selected Finder Items"
Drag it over to the right side of the window.
Now find "Preview" on the left.
You're looking for "Convert file to..." (or something close)
Drag it to the right side.
File>Save as Automator Action, it's the default so don't change anything...
Name it "Conversion" or whatever.
Now, select/highlight all of files you want to convert, right click one of them (still with all of them highlighted) and go down to "Automator">"WhateverYouNamedIT".
This should start the workflow and it will probably take a while to do 100+.
Sorry for the inclarity, but like I said, I'm at work...on a PC. Ugh..
It would be easy to do it in the command line, but if Kokapelli's command were to be interrupted by something (Terminal quitting or whatever), you'd corrupt most of those files. If you don't have Tiger, this may be your only choice however...