You are definitely running low on disk space.
I think the problem is related to whatever is going on with your Photos or iPhoto library. In the sidebar, it says zero bytes used for Photos. But you seem to have two iPhoto libraries on your desktop. The "System" value appears to be calculated by simple subtraction. It takes your total disk space, subtracts out everything that it can account for, and calls the rest "System". But it seems to be having trouble accounting for those iPhoto libraries on your desktop. Plus, all that extra space for iPhoto libraries is likely what is consuming all of your disk space.
Before doing anything, make sure you have a good backup of your system and your photos.