Medium and large images show small

Hi:
I have read a lot on images, and still cannot get it right for some reason.
I am working on my computer (localhost). I've added one of the Small (100x100 px) images to all of my products. There are approx. 50 products.
Those images are saved automatically in "root/images/ folder" ("main" directory).
Then I added, by dragging and dropping, Medium images (250x250 px) to "root/images/medium" folder, and Large images (500x500px)- to "root/images/large" folder.
The result is that individual product page has a small image on it, not Medium size. When I click on "larger image", the same small image appears in a lightbox.
Am I correct to save small images in general root/images folder? may be it should go into root/includes/templates/MySite/images folder? The same question about Medium and Large images? Should they go into root/includes/templates/MySite/images folder or root/images is ok?
Just don't know what else I can do. One site mentioned that medium images should be in root/images folder. I tried that, no good.
I did change image sizes in admin/config/image size, except for pop-up large images, as I am not sure where to change those. I did tried to change css? or was it php? file, but that didn't work either.
Will appreciate help! Thank you.

You did the right things, according to your description. You mention a "lightbox" which is not stock Zen Cart, so I have to ask what else is customized. What can you tell us about your customizations?

only lightbox which i downloaded from zen cart plug in after i tried everything what i could with no results. without lightbox there was a pop-up window with the same small image. I tried to add Medium image to product description, and then on product page the Medium image appeared, and the same Medium image appeared in the pop-up window.

What sizes have you set for various kinds of images (in admin > Configuration > Images)?
What template do you have active?

small im width/height - 100/100
heading image w/h - 57/40
subcateg w/h - 100/100
calculate image size - true
image req - true
image - shop cart -1
image - shop cart w/h -50/40
category icon -product info pages -w/h -100/100
top subcateg w/h - 150/150
product info - w/h -250/250
product info - image med suffix - _MED
same for large - _LRG
product listing w/h - 100/100
product new listing w/h - 100/100
image - new product w/h - 100/100
the rest is 100/100

You're doing it the old-fashioned, no add-on way which is fine - that's what I do. If it all shows up as small then for some reason it's not reading the medium and large and associating them with the small. What is your naming convention for all three sizes? That can throw it off. You're not giving all three images the exact same name are you, or giving the medium/large images names that don't follow off of the small?

Wrong: fuzzynavel.jpg (in all three folders)
Wrong: fuzzynavel1.jpg (in the small) fuzzynavel2.jpg (in the medium) fuzzynavel3.jpg (in the large)
Right: fuzzynavel.jpg (small) fuzzynavel_med.jpg (medium) fuzzynavel_lrg.jpg (in the large)

took one image as an example and checked it in all folders.
small size, which is in "root/images": product_drkblue_SM.jpg
medium size, "root/images/medium": product_drkblue_MED.jpg
large size, "root/images/large": product_drkblue_LRG.jpg.

just for reference: I tried first without _SM, the same result, so now i have it for clarity since there is no difference anyway.
is there some other place where setting can be changed?
I made first changes in php file as was described in my first post (for pop-up window size), but then replaced it with the original settings, i guess. But anyway, the medium size is not showing, as well as the large size.

line1:

took one image as an example and checked it in all folders.
small size, which is in "root/images": product_drkblue_SM.jpg
medium size, "root/images/medium": product_drkblue_MED.jpg
large size, "root/images/large": product_drkblue_LRG.jpg.

just for reference: I tried first without _SM, the same result, so now i have it for clarity since there is no difference anyway.
is there some other place where setting can be changed?
I made first changes in php file as was described in my first post (for pop-up window size), but then replaced it with the original settings, i guess. But anyway, the medium size is not showing, as well as the large size.
The ones where you have _SM will definitely not work. All extra images must start with the exact name of the original image. So if you have image_SM then when it goes to the images/medium folder it is looking for anything that starts with image_SM and image_MED does NOT fit that criteria.
(Also the extensions must be the same. .JPG is not the same as .jpg)
If you did it image and then image_MED image_LRG it should have worked. Assuming you made the change from image_SM to image correctly it should have worked unless you've done something to your code. Maybe try once more but with a new product, new image. Call the small image.jpg, and put it in root/images via the add-a-product page. Then drag a medium image called image_MED into the /root/images/medium folder and a large called image_LRG into the root/images/large folder and make sure they all end in .jpg and not a mix of .jpg and .JPG. If that doesn't work then it's hard to diagnose since we can't visit your site and don't know how/what you added/deleted.

Thank you!!!
Yes, it works now. That was the reason.
It is so great to be able to get help from experienced people!
Again, thank you!