Step 1: Navigate to the social / entries tab on the bottom navigation when you're editing your sweepstakes or leaderboard.
Step 2: Upload the image here
Troubleshooting: If it's not working
Most likely your image is too large for Facebook. If so, please compress the image down to a smaller size. We also recommend jpeg for the compression.
After you change your Facebook share image, you need to ask Facebook to "un-cache" the image. Go to the Facebook debugger and enter your contest URL inside the debugger to do this.