Thousands of images saved to my computer unintentionally

Ugh. I don't know how this happened. I configured Mail and it downloaded all my emails to my computer (and I have a lot, saved over several years). But when I look in my All Images folder, I see that every attachment I ever received in an email is there, every single image or logo that is part of a person's email or part of their email signature is there, even people's avatar pictures from online forums that I have replied to, for some reason, those pictures have also gotten saved in my All Images folder. And there are multiple copies of all of these images, literally thousands. .

How did this happen? This was not my intent, to clutter up my hard-drive with all these pointless images. I can go and delete them all and I will, but how did this happen, and how can I prevent this from happening again as new emails come in?

The all images folder isn't stored images.... it is a history folder.
There is nothing for you to worry about.
If you right click on your pictures folder, and select get info, you will get a real idea as to what space you have used up.
You can also use disc inventory x to see the whole HD.

Disk Inventory X

Ok so I don't need to clean out this folder? That's a relief. I guess the purpose is in case I wanted to save one of the images?

No, there's no folder to clear out.
They are not duplicates, however they are items that exist on your HD, and if you were to delete any of the items, they would no longer show up in the today, yesterday, past week, all images, all movies, or all documents searches.

Best is to look in you pictures folder and see what you have in there.
If you want to be more thorough about knowing what's on your HD, use Disk Inventory X.

Me personally, I use the right click get info method on my folders in my home folder every now and then just to get an idea as to what I have stored, and how large the files are, and if ever in doubt, just run through the things that look out of place.

For me I prefer not to have anything under search for, as I use spotlight or finder window search to find things, assuming I don't know where they are.
To get rid of the search for items, if you want to, you can go to Finder>preferences>side bar and uncheck the items.

Generally, photos, images, documents, etc. take up very little HD space, even when there are 10's of 1000's of items.
It is the music and videos that takes up the better chunk of HD space.