Hi Pete
The directory structure will be a breeze and cause no headaches if you give it some prior thought. You can find lots of simple, and not so simple, articles about this if you do a search on the web so I will not repeat it here.
I agree that your image naming means something to you but you need to consider the
SEO and accessibility considerations.

I could name this image "sheep.
gif" but would it not be more descriptive to name it "brownface_sheep.
gif". There are limits of course but I think "brownface_sheep_with_big_eyes_and_fourlegs.gi f would be total overkill
If your image is a version of your site logo then why not name it as such??
Dont forget to use the image alt tag to good advantage also, this should give a good description of the image. Once again plenty of advice about this with a web search.
Hope all this helps and good luck with your web site, remember that a little forward planning can save a lot of grief later on.
GT