Zero space on my macbook air?

I got a 128GB 1.6GHz MacBook Air last year for school, thinking there was no way I could ever fill up a hard drive with that much, so why shell out even more for 256GB? I'm sorely regretting that now, but I don't know if I should be.

I have no space left. Zero. Zilch. I'll let you see for yourself.

The only real culprit I can find is my Photos library. It is nearly 20GB, which just baffles me because I have just over 2000 pictures, and mayyybe 20 videos in there. Of those videos, the largest one is 1GB, and I know there's one or two that are upwards of 500MB, but most of them are under 100MB. I'm not a computer expert but I know that those should NOT be taking up that much space. I also don't understand why in Finder it says my Photos library is 20GB in size when the usage bar says my photos only take up 8GB of space.

When I go into Finder, hit "All My Files" and sort by size, I have 9 - you read that right - nine files bigger than 100MB. I have 1462 files in total, and 950 of them are 10KB to 1MB in size. There is no way my files could possibly taking up that much space. I have downloaded maybe 3 applications on this computer besides what came on it - one was Chrome, one was SelfControl, and the other is the Kindle App (which I wanted to use for my textbooks this year but can't until I get this Mac cleaned up).

Can anyone tell me where in the world this 'other' is coming from? It's driving me absolutely crazy. For the record, I've already deleted old iPhone backups and the one I have is not very big. Does this have anything to do with the fact that I'm still running on Yosemite and not El Capitan? Appreciate the help in advance!