The true answer is to follow your heart, but here are two rules I have discovered while making the network:
1) Don't try to be perfect.
2) Only tag using tags you would actually type in and search with.
Think about when you would want those images to appear–if you might want to find that third image when you type 'cat', then add 'cat'. If you typically think you would type in colours or patterns of light like 'sunset' or whatever, then try adding those tags.
Lots of people get caught up in trying to adding a huge number of 'descriptive' tags, but imo they are mostly a useless time sink–you can already see a file, so you know what it depicts. The worth of tags is in searching.
If you are tagging on your 'local tags', things like 'inspirational' or 'wallpaper' can be very useful as well. I also use some like/dislike ratings to make quickly-editable pools so I can quickly find my art images for posting and so on.