How can I restore a Whatsapp backup while retaining stuff from after the backup? For a number of reasons, I just deleted my entire Whatsapp images folder by accident. I backed Whatsapp up yesterday morning, but yesterday night I had a sentimental conversation with a friend about some issues I've been having, and I don't want to lose that either. How can I go about doing this without losing that conversation?
You actually could have a perfect backup that was automatically created after night conversation and before accidental erasing. Whatsapp creates local backup files at 2 am (up to 7 states), also it saves data to Google Drive (if configured). Check dates of files in sdcard/WhatsApp/Databases as well as in Drive interface (again, if Drive backup was configured).
If automated backup was not performed in a desirable timeframe you will need to mess with local backup files and apps reinstallations to get your old, yesterday state, copy images to safe folder and restore a backup with night conversation.
Official FAQ states that you can force an app to restore a particular backup file:


  To restore a less recent local backup
  
  Your phone will store up to the last 7 days worth of local backup
  files (Google Drive will only have the most recent one). If you wish
  to restore a local backup which is not the most recent one, you will
  need to do the following:
  
  
  Download a file manager app.
  In the file manager app, navigate to sdcard/WhatsApp/Databases. If your data is not stored on the SD card, you may see "internal storage"
  or "main storage" instead of sdcard.
  Rename the backup file you wish to restore from msgstore-YYYY-MM-DD.1.db.crypt12 to msgstore.db.crypt12. It is
  possible that an earlier backup may be on an earlier protocol, such as
  crypt9 or crypt10. Do not change the number of the crypt extension.
  Uninstall WhatsApp.
  Install WhatsApp.
  Tap Restore when asked.
  


Sources:
https://faq.whatsapp.com/en/android/20887921/?category=5245251
https://support.google.com/drive/answer/6305834?co=GENIE.Platform%3DAndroid&amp;hl=en