G Mail Photos

My Sister put Her SD Card from Her Old Phone into Her new Phone.

She was able to see all Her Photos on the SD Card.
But all the Photos She Deleted from Her Old Phone are back as well.

I Told Her your Old Galaxy J7. Phone Must have Backed all your Photos up to your G Mail Account.
And then it put them Back on your new Phone.

I do not know if I am Right?

So I go on my DeskTop Computer and LogIn to Her G Mail Account then Click on the Top Right where the Little Squares are.

Then I go down the List and Click on Photos.

All Her Photos Open.

But if you put your Mouse over a Photo on the Top Left of the Photo is a Circle with a CheckMark in it.

What is the CheckMark for and how do wee Delete Photos from here?