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.