About CC licences

This post is about licences: I've noticed that many stock providers have licenced their stock under CC BY-SA -licence.
Which says:

"This license lets others remix, tweak, and build upon your work even for commercial purposes, as long as they credit you and license their new creations under the identical terms. This license is often compared to “copyleft” free and open source software licenses. All new works based on yours will carry the same license, so any derivatives will also allow commercial use. This is the license used by Wikipedia, and is recommended for materials that would benefit from incorporating content from Wikipedia and similarly licensed projects."

My attention is on the sentence "All new works based on yours will carry the same license, so any derivatives will also allow commercial use."

I've been reading the licence -text, also the legal version of the text. Have to say that this is a bit hairy stuff.

My question is that if I make a piece of art, for example a photomanipulation, and partly use stock which is licenced under this SA-licence, what does this mean for my artpiece? Does it mean than that anyone can copy or/and sell it? Do I have an obligation to share it? The licence says that my artpiece will also be under SA-licence, because I've used stock which has been put under SA-licence.

I love the idea on SA-licence: I use Wordpress, which to my understanding is based on this licence, as is the whole Open Source -sharing. I love it and applause for it.
But concerning individual artpieces the SA-licence raises questions. And yeah, I've been reading the legal text where it does something about the copyrights in the first section where the definitions are:

Section 1 - Definitions
d. Copyright and Similar Rights means copyright and/or similar rights closely related to copyright including, without limitation, performance, broadcast, sound recording, and Sui Generis Database Rights, without regard to how the rights are labeled or categorized. For purposes of this Public License, the rights specified in Section 2(b)(1)-(2) are not Copyright and Similar Rights.

Section 2 – Scope.

License grant.

1. Subject to the terms and conditions of this Public License, the Licensor hereby grants You a worldwide, royalty-free, non-sublicensable, non-exclusive, irrevocable license to exercise the Licensed Rights in the Licensed Material to:
   1. reproduce and Share the Licensed Material, in whole or in part; and
   2. produce, reproduce, and Share Adapted Material.
2. Exceptions and Limitations. For the avoidance of doubt, where Exceptions and Limitations apply to Your use, this Public License does not apply, and You do not need to comply with its terms and conditions.

This implies that an individual art piece is ruled out of the licence influence based on copyrights, even though material under SA-licence would have been used as stock. But is it so?

Any thoughts on this?

I just would like to know has anyone any experience or legal viewpoints on this so that it would be clear what does it mean for the art piece if someone uses stock which is licenced under SA-licence.

Ps. I found this: How to use a work with a Share Alike licence