It's not possible to get it back through WhatsApp, since they don't store images on their servers anymore after you've downloaded them once.
You can try and use your Photos app and then through the menu look at your Trash; deleted pictures should still be there for 30 days if I'm not mistaken.
Another option might be to check a photo backup service if you set one up (most suggest to automatically set it up on first access of the app, so could he set up without you remembering it). Are you syncing your pictures with Google+ or Dropbox perhaps? There the picture might still be accessible. If you don't have any of these set up yet, it might be a good idea to do so to avoid this situation in the future (please take privacy and battery concerns into consideration).
If all of the above fail, your last bet would be to remove the SD card from your phone and put it in a laptop/desktop (you will probably need an adapter for that). From there you should run some recovery software on the card.
Deleted items on the SD card are actually not deleted, but merely 'forgotten about' until something new is written over them. That means this method will have a much higher success rate if the deletion was recent.
If you need to resort to this solution, please search this forum with a query like "recover images on SD card" for much more thorough answers than I can give you here.