I have Hero3+ Black Edition.

I can't understand if Protune is for video only or may be applied for photos?

The manual says it is both for video and for photo (page 35):

High-Quality Image Capture

Protune’s high data rate captures images with less compression, giving content creators higher quality for professional productions.

Film/TV Rate Standard

While shooting in Protune, you have the option of recording video in cinema quality 24 fps to easily intercut GoPro content with other source media without the need to perform fps conversion.

But at the same time their site says that Protune is for video only:

To record Protune footage, you’ll need to turn Protune ON in your camera’s settings menu.

What for is Protune? Photo? Video? Or both?