There are two ways to delete a picture from your Gallery.

Enter the Gallery by tapping the Blue icon with the pictures .

To delete a picture from the Gallery menu, select the picture you want to delete and drag it to the green recycle bin on the bottom left corner.

When you are asked to confirm, tap yes.

You can also select the picture first.

Then tap on the Eraser at the bottom left corner. Once you see the Green recycle bin, tap on it.  

Tap Yes when asked if you want to erase everything and start over.