Can someone explain this to me??

ok so line the pics up in your folder tht u want to đăng tải side bởi side, Then, hit the "ctrl"button and at the same time click on ur 1st pic then keep clicking while pressing ctrl, it should look like this so there should be frames around the pics u selected. hope this helps !

When bạn choose the hình ảnh bạn must press the shift button to select many các bức ảnh at the same time
Try it, It works with me :)