If I left click on a photo I get this message "There are two actions (Cut,Delete) that want to use the same shortcut (Shift+Del). This is most probably a bug.." and Gwenview is not opening, but as soon as I click OK on the message Gwenview open the photo. I am using Mint 18.2 beta in VirtualBox.
Looks like a duplicate of https://bugs.kde.org/show_bug.cgi?id=377880, which is ultimately a dupe of https://bugs.kde.org/show_bug.cgi?id=347373, which has been fixed in a newer version of Gwenview than what your distro is providing. *** This bug has been marked as a duplicate of bug 377880 ***