If your Blogger site is showing missing image boxes one day its probably because you deleted photos from your Picasa account or even Google+ account. You can restore these, but only within the first 60 days. Otherwise they are gone forever.
To restore the missing photos from blogger. Login to your Google account and go to Google Photos. On the left sidebar, click More (this could be represented by 3 dots). A side menu will open and you will see a Trash folder. You can restore any phoos still in your trash. Select the photos you want to recover and click Restore. If you don’t see photo’s there, Google has already deleted them and can’t be recovered this way.