In addition to the other answers, the following directory contains deletable system images on a Mac for Android Studio 2.3.3. I was able to delete the android-16 and android-17 directories without any problem because I didn't have any emulators which used them. (I kept the android-24 which was in use.)
$ pwd
/Users/gareth/Library/Android/sdk/system-images
$ du -h
2.5G ./android-16/default/x86
2.5G ./android-16/default
2.5G ./android-16/google_apis/x86
2.5G ./android-16/google_apis
5.1G ./android-16
2.5G ./android-17/default/x86
2.5G ./android-17/default
2.5G ./android-17
3.0G ./android-24/default/x86_64
3.0G ./android-24/default
3.0G ./android-24
11G .