I don't have a picture sharing site account to use, so I can't post any screen shots. Hopefully, I can describe it well enough.
Say you have a user account called 'bob' on your Mac. On your Mac, bob is an admin user and therefore has access to your entire Mac. If you log into your Mac as bob, you can see everything.
Nothing changes if you log into your Mac from a Windows machine as bob. No matter how bob logs into your Mac, directly to the machine, from a windows machine on you network, or from a dumb terminal in the basement of a hangar on Area 51, bob will have the same access to your Mac.
In the File Sharing options, when you select the account for bob, the entire account is shared. You essentially said, "let 'bob' log into this machine using SMB."
If you just select the option to share files via SMB, you can set access to specific folders for particular users. You can even make up new users that don't have an account on the Mac, but can access folders that you specifically give them access to.
So, for what you want, don't select an account to share via SMB, just select "Share files and folders using SMB."
Now, at the bottom of the file sharing dialog, Add the folders you want to share. Choose the users you want to have access to those folders and set the type of access, Read & Write, Read Only, or Write Only. If you want to create a username and password to access those folders, without having an account on the Mac, Add a person and choose "New Person." That person will be a "Sharing Only" user. You can't log into the Mac as that user, but you can access the specific folders you have set up.