How to get a confirmation when deleting images? I just bought the LG G4 (Android Lollipop) and I'm pretty concerned that there's no confirmation "dialog" or something when we delete images in the Gallery.

Something like "Are you sure?".
Because now when I press the trash button to delete image, I have like 2 seconds to press Cancel if I don't want my image to be permanently removed.

This can be problematic if I missclick on the screen or if I let someone use my phone...

My step-father used my phone quickly just to check-out, he took a picture then wanted to delete it.. Then I don't know what happened but my previous photos were removed. I think he deleted all the photos by tapping too fast?

Well, I searched everywhere and I didn't find anything to have more security at least.
I have a G4, and you're right. The Gallery app doesn't have a confirmation message, just a floating dialog that has an Undo button which disappears after a while. 

I'd recommend using another app instead. I use QuickPic. When you click the delete button, a popup with a delete icon appears. It serves as sort of a confirmation message. If you click the button, the picture is deleted. Otherwise, you can just cancel the operation by clicking the soft-key back button.