I had about 500Mb free space on my computer
Was this before you added your holiday pictures??
What exactly have you done between the 500Mb free and 15Mb? Note that one CD stored on the hard drive is worth between 600 and 800 Mb. 500Mb ain't a lot of space, you can also have log files which grow during a session and take up room.
in a terminal, if your disk is anywhere near 95% full you need to clear it out; pictures, videos, music files, unused apps, etc.