The image should have the following dimensions:

Width: 140px

Height: 140px

To replace the default no photo image, upload the new image to Admin area > Appearance > Edit theme > Graphics Copy the link to the image.

Then add the style below to the Edit CSS tab:

ow_avatar_console{
background-image: url('images/no-avatar-big.png');
}

Replace images/no-avatar-big.png with the new image link copied from Graphics.

Click Save.