I saw nice album pictures on some screenshots, where can I add them?

Album art is added for all songs scanned and added to your music database, see Configuration > Music > Database

Or, you may manually add a folder.jpg image to your album folder in Windows. If such a file is detected by MediaPortal, it will be displayed as the album art when any song in that folder is playing.  Note, you will need to have each album stored in a separate folder.

To use an image for a single track, name the image file the same as the audio file but with a .tbn extension. This acts as a thumbnail image that will be displayed when the file is playing. For example, given a song called song1.mp3, name the image for this file song1.tbn.