Here I am sharing the method of saving your picture in WhatsApp:
First, Open Settings
Second, then you will scroll down and will tap on Privacy and then Photos
Now, turn on WhatsApp Switch
In this step, you will open 'WhatsApp' and you will tap on chat where you shared image
Fifth, tap on that particular image
Once, image is in full-mode that click on share icon
After successful completion of sixth step, tap on Save Image
So, it is the simplest process that I shared with you above, and I hope you found it quite helpful.