Prior to OSX 10.9.2 I often needed to take screen shots for WordPress or documents, and therefore it is most useful to have them as JPG format.
By default the OSX utility Grab saves files in TIFF format, so you have to open them in Preview and then Export as JPG. Far too much work!
Opening a terminal window and typing
defaults write com.apple.screencapture type jpeg
sets the default save mode to be JPG. (jpeg and jpg are the same thing)
The file type options are:
- defaults write com.apple.screencapture type png
- defaults write com.apple.screencapture type pdf
- defaults write com.apple.screencapture type jpg
- defaults write com.apple.screencapture type jpeg
- defaults write com.apple.screencapture type tif
- defaults write com.apple.screencapture type psd
For the change to be effective you can then do a restart or use the command ‘killall SystemUIServer’ if you wish.
After 10.9.2 something changed to prevent this from working so the majority of my screenshots were done using Skitch, or latter just by using the Evernote Web Clipper extension in Chrome.
BigR's Weblog 
this method just doesnt work for my macbook at all 😦
currently running OSX Mavericks 10.9.2
HELP PLEASE!
And you have done the restart?
If you can’t get it to work, use Skitch perhaps?
This doesn’t work in mavericks. apparently used to work in previous versions
Doesn’t work on 10.9.5….Dont any of you Mac guys ever stay current on this stuff? How do you do this now?
I just use Skitch now, I’ll update the post with that information as it makes it simple – even if you choose not to use the Evernote backend.
This doesn’t work in el capitan public beta 5 😦
I think quite a few things have changed in el capitan and they may have moved away from the com.apple type structure for preferences. I haven’t got proof though – it is just a suspicion after trying to play around in the backend with other things as well
This method will never work in Grab since this is a stand-alone application; the “solution” presented wil only work for the screengrabbing function, presented in OSX itself (for instance using the cmd-shift-3 or -4)