What the heck!! I had done this graphic in an Excel spreadsheet, so my first thought was that it was something to do with the background color in there. After several attempts at changing the spreadsheet background color and the graphic background color, I ruled that out as the cause.
Searching for the problem led me to some posts about a change in Google+. According to a Google+ Help Page, "When you add new photos, Auto Enhance makes subtle adjustments (like removing red-eye and improving lighting) to help them look great". The page also said that Auto Enhance is on by default.
To turn the feature off, execute the following steps:
1. Go to your Google+ home page.
2. Hover your mouse over the Home menu in the upper left corner.
3. Select the Settings option.
4. Scroll down to the Photos section and set Auto Enhance to Off. I also unchecked Auto Awesome and checked another box in the Photos section to upload my photos at full size.
OK, so here's the really crazy thing. I was not a member of Google+! In order to change these settings and return Blogger image uploading to normal, I had to enable my Google profile for Google+. After doing this, I went back to the Settings menu, selected Profiles and Privacy and locked down everything I could.
There is an option at the very bottom of the Settings page to disable Google+, but I'm not sure what effect that would have on the settings I changed. I decided to leave it for now, but I'm not at all happy with the way Google forced these new settings and services on us. It would be much more user friendly to default new features to "Off" rather than make users spend a lot of time trying to figure out what changed.