I have some simple/stupid questions RE my Desk Top

This is RE the pictures I put on my Desk Top..and the choices I have.

I know how to do it. I know how to size it....choose it from the list...."stretch"---"center"---or "tile"....
I also know that, if i come across a particularly nice photo of something--i can right-click and choose it as my
"Desk Top" picture...But--it seems a one-time thing when I do.

SO??? Why am I writing to you all "Gurus???"

--My list of available DT (Desk Top) pictures seem to be pre-programmed. I sure do not know where they came from!
There are pictures there that are NOT pictures at all. Weird, computerized images. I would NEVER choose one of them!
OH! There are some nice ones--but I want some other nice ones of my own choosing.
This function, of course, replaces the picture that was there before. Then--I never seem to be able to get it back again.
WHERE did my Bluebird sitting on a fence post go????? I LOVE that one!!! GONE!

Questions:

--If I want to change the picture I have on my DT to a more seasonal one--WHY does the one I had before go away forever?
It won't IF it was on the list that came with he computer. But if I had a picture i had chosen from, say, DG--as my DT--
It is nowhere to be found again for life!!!
Lets assume that there is only so much space on the Pre-list that is there to choose from
--How could I delete the DT picture options (on the list) If I NEVER, EVER plan to use them?
And--How could I make sure that any pictures I have chosen that I just LOVE for my DT--say from the DG Photo Contest--
that they will be available on the "list" one gets when you click on the Desk Top options. They just are NOT there!!!!
--WHERE do they go?
--HOW can I find them again? I would think that once I have chosen a picture--and then change my selection--
that the no longer wanted picture should fall into the list of available pictures for my Desk Top for a future selection. YES?

This leads me to ask---HOW does one DELETE the DT pictures offered that I have absolutely NO interest in?????
WHY are they there---FOREVER?????? I want to get rid of them! I want the allotted spaces to be filled with the many
other pictures I have chosen--but put them aside for seasonal reasons. Who wants a snow scene in July?
I want "comfortable" pictures--as I choose. I do not want to lose them when i choose another picture.
I want my Blue Bird back!!!! NOW--Don't all of you send me that picture! One of you may!

The main question here is--
HOW do I get rid of the per-programmed pictures and replace them with pictures of
MY choice--and have them stay there so I can chose from them again and again in the future??

Same goes for all the Screen Saver pictures! Most of them are just SOOO....bizarre!

I do not know the "guts" of a computer too well. Can anyone offer me simple solutions to this? Be basic about this, PLEASE!

Thanks! Gita