The idea is very simple and I think I should have used it all along.
Whenever you have an idea. Just launch your text editor and write the idea. The images from which you got inspiration will be on the screenshot you take of the whole screen.
Simple and very effective.
Further I wonder if you can take a screencast while creating music. That way you can store the feeling you had. Quickly capture everything that happened that day and go back and improve. Find out what others thought was good that one perticular day and so on.
What I would need in Linux is a transparent drawing window. On this I could draw red arrows and underlines for any program I like.