Note that the author licensed it under CC BY-SA 2.0 (that’s what it says on the Flickr page), while the blog post that made use of this image specifies CC BY-SA 4.0 instead. It’s allowed to license contributions to this image under a newer BY-SA license, but not the original work (only the author may choose to do this). So unless the blog post author got the permission from the image author, their attribution is not correct, and you should assume that the image is licensed under CC BY-SA 2.0.
Your attribution isn’t sufficient.
For CC BY-SA 2.0, a correct attribution (for using the original image) contains (if available):
- name (or pseudonym) of the author
- title of the work
- URI of the work
and you must specify
- the URI of the license (or include a copy of the license)
So it could look like this:
Chapter cover art: tricorderunbox4 (https://www.flickr.com/photos/bojo/4078685614) from Bobbie Johnson, licensed under CC BY-SA 2.0 (https://creativecommons.org/licenses/by-sa/2.0/)
or like this if hyperlinks are possible:
Chapter cover art: tricorderunbox4 from Bobbie Johnson, licensed under CC BY-SA 2.0
The ShareAlike (SA) element of the license requires that derivative works have to be licensed under a compatible license, too.
Deciding whether or not a work becomes a derivative work isn’t always easy. And it’s not relevant if the image got modified:
- A work could become a derivative even if the image is used unmodified.
- If a work doesn’t become a derivative, you could still edit the image (it would only affect the license of that image, not your whole document).
In your case, I think that the thesis doesn’t become a derivative work, as the image doesn’t affect the rest of its content in any way. So it should be fine to not license your thesis, or to license it under a different license.