If Pixlr Desktop was working until recently, but is now crashing on start up, chances are, your preferences have become corrupted.
Clearing out your preference files will return Pixlr to a default condition and allow you to restart Pixlr without crashing.
REFRESHING YOUR PREFERENCES ON OS X
1. Close Pixlr if you already have it open and open the FINDER.
2. From the main menu click on the GO menu. Press and hold the ALT key and select LIBRARY from the menu list.
3. In the LIBRARY folder, select the APPLICATION SUPPORT folder and delete the PIXLR folder in there.
4. Return to the LIBRARY folder and select the CONTAINERS folder. Delete any folders with PIXLR in it's name, if any.
5. Return to the LIBRARY and select the PREFERENCES folder. Delete any folders with PIXLR in it's name, if any.
6. Restart Pixlr.
REFRESHING YOUR PREFERENCES ON WINDOWS 7 AND 8
1. Close Pixlr if you already have it open and open an Explore window (WIN + e)
2. Copy and paste the following path into navigation field of the Explore window. Don't forget to edit the path to reflect your USERNAME.
3. Delete all folder(s) with PIXLR in their label.
4. Restart Pixlr.