really time machine doesn't do backups in a day week month concept the way we normally think of it. What it really does is a versioning backup.
Everything gets backed up once. Changed files get updated. As new versions of a file are backed up. old version of the file are deleted. If a file doesn't get changed there is no need to back it up again.
so when it talks about hourly, daily & weekly backups - what it is really referring to is the previous versions of a file.
Imagine you have a 100mb file, A 100GB home directory, and a 200GB drive for backups. Every 2 hours you make a change to that 100mb file. Every time you do TM backs up the whole file all over again. After 10 changes you have 10 version of the file. This is a GB of space devoted to that one file. It is obvious to see that it wouldn't take long for the one file to fill up the backup drive (when you also add in the home directory) since every time it is modified it is backed up.
To prevent this from happening, TM drops old versions for you so you hard drive is constantly dropping duplicate versions - the oldest removed first.
But something that is only on there once, never gets removed. it is only the changed files. and with those it is the oldest first.
So in your example - the pictures are never removed.
Does that make more sense?
This whole "deleting older version" must happen only when the disk begin to fill up.
I mean, I have clearly different versions of the same photoshop file (same name, same directory) in my Time Machine Backup.
Also, I think you don't directly answer the question.
Lets make an example.
We are Monday the 1st. We have a "weekly backup".
Wednesday, the 3rd, I create a new file. Time Machine does a backup one or two time. Thursday the 4th, I delete the file I created Wednesday. Time Machine does a backup, and on the date "Thursday the 4th", the file is not there in the Backup.
Then, some time after, Time Machine begin to keep "Weekly backup" of that period. Does a backup called "Monday the 1st", then another one "Monday the 8th". Technically, in backups that are made these two days, the file I created in the middle of the week and then deleted, does not appear in the standard backup. But is Time Machine still keep the file that has been there just for a day in the backup of "Monday the 8th". Because at this time, the backup called by this name is not really what my harddrive was in Monday the 8th. Or in fact the backup is in reality a "Sum of the last week".