This was on MacOSXHints.com a few days back, but it’s handy enough to repeat: Convert image file types via Terminal using sips. To convert a .tiff to a .jpeg:
sips --setProperty format jpeg --setProperty formatOptions normal
bluetooth_broke.tiff --out bluetooth_broke.jpg
There’s a line break in there just so that it displays correctly here… You can convert a dozen different filetypes, play with it for a while to figure it all out.