Find WhatsApp photo's sender in Gallery I lost my mobile so I lost all numbers. I really need to find my best friend's number. I have a photo on my hard disk which he had sent me through WhatsApp before.
Can I find his number through its details? How can I find the WhatsApp photo's sender in the gallery by its details?
you cannot find the sender details from the photo. There's no EXIF data added to the photo which can track its sender. So, that's the answer to your question.
Next, you do have other ways by which you can likely find the number:

If you sync contacts to any cloud service (such as Google contacts), you can check there and find out the number.
If you backup WhatsApp to your Google Drive, then you can install WhatsApp on another phone using the same phone number and then restore the backup from Google Drive. The backup will also bring in your old chats and hence the phone number.
Install WhatsApp on your phone and wait for your friend to message you. You'll get the number.