Well you asked a bigger question than you might think...the short answer is no there isn't an optimal number as you've presented the question.
It's apparent it's important enough that you would like some optimization but not important enough that you care to give many more details. I'd say don't worry about it if most other variables remain unchanged and all you want to experiment with is number of files in an NTFS folder. because the amount of optimization will be hardly noticeable to you if you are simply using explorer. If you really don't want to change anything I'd say you should focus on things like disabling windows disk indexing which is very intensive bookeeping which slows down your PC (though there might be certain reasons to use it). maybe a great alternative for you is using a program like picasa, which is really good at handling large numbers of image files and accessing them efficiently.
without other details like how often you update files, how long before migration, how often are they accessed before migration, exactly why are you accessing such a large repository of image files using NTFS explorer, etc., all sorts of other details that would help in optimizing your answer, it's hard to say what would best help you. if you really want to make a serious stride in optimizing file access set up an inexpensive RAID configuration. good luck hope i said soimething that may be useful.