It would be nonsensical if you have more rights to someone else's work than to your own work. Therefore, you are likely able to quote your own work.
In the context of CC-BY-SA, it is important that not only do you have the right to publish the resulting work, but that the resulting work is in fact correctly licensed under the CC-BY-SA, so that others may share and modify it. It may therefore not be adapted from non-CC-BY-SA works.
This requires that you properly quote your own work as a separate work. When relying on copyright exemptions such as a right to quote, this may require the quotation to be de minimis, and properly attributed. For example, including a high-res image might not be covered by such copyright exemptions, whereas a low-res thumbnail or a cropped part with a link to the original work may be covered. Whether technologies like HTML
<iframe> tags embed content or merely reference it is an interesting but definitively not resolved question.
So do make sure that you only exercise rights you would have to another's work, and do not make use of the rights you have as the creator of the quoted work. Otherwise, it would not be unreasonable to assume that you had in fact licensed the quoted work under the CC license, because otherwise the resulting work (containing the “citation”) could not be licensed under CC-BY-SA.
You could perhaps also license the work so that it can be included in CC-BY-SA works, without falling under the CC-BY-SA itself. For example, by using a CC-BY license. You can then embed the work in its entirety, without relying on a right to quote. However, you have to make sure that the embedded work and the CC-BY-SA embedding work are clearly separate works. Here, this might be rather problematic because the combination of these works clearly does not fall under the CC-BY-SA in its entirety. I think this could be fine if you can clearly separate the licenses (e.g. in a book with CC-BY-SA text but non-free illustrations). But in the context of creating content on a web site where all user submissions are CC-BY-SA, I don't think this would be possible, so that you would have implicitly licensed the embedded work under CC-BY-SA as well.
- You can't arbitrarily exclude parts of a work from the CC-BY-SA if you agree to license the whole work under the CC-BY-SA.
- Self-quoting is OK, but you are then bound by the restrictions of applicable rights to quote or fair use rights. Do not make use of any rights you have as the creator of the quoted work.
<img src="//privatesite.foo/myimg.png">is legally quite different from uploading your image to someone's server.)