Image credits should be added

[javierev]

The old credit info was:

Images created by using photos by Maribel Tixe (portrait), Jonathan Rubio (mailbox) and Javier Estévez (keyboard).'

Now it should be server information in the JSON, something like:

{
  ...
  imgUrl : 'img/image.jpg',
  imgCredits : 'Image by <a href="...">Name</a>.',
  ...
}