Using images with CC-BY-SA license in slides or a thesis
Including CC-licensed images as discrete elements in your work does not require the overall work to be CC-licensed. Reasoning 2 is correct here. 

(See a similar discussion on opendata.se)

The BY-SA license works in terms of "collective works" and "derivative works":


  1a. "Collective Work" means a work, such as a periodical issue,
  anthology or encyclopedia, in which the Work in its entirety in
  unmodified form, along with one or more other contributions,
  constituting separate and independent works in themselves, are
  assembled into a collective whole. A work that constitutes a
  Collective Work will not be considered a Derivative Work (as defined
  below) for the purposes of this License.
  
  1c. "Derivative Work" means a work based upon the Work or upon the
  Work and other pre-existing works, such as a translation, musical
  arrangement, dramatization, fictionalization, motion picture version,
  sound recording, art reproduction, abridgment, condensation, or any
  other form in which the Work may be recast, transformed, or adapted,
  except that a work that constitutes a Collective Work will not be
  considered a Derivative Work for the purpose of this License. (...)


An image reproduced as part of a larger document ("see Figure 17...") falls under 1a as a collective work. You have to comply with the formalities of the licensing (4a, identify the license, 4c, identify the author), but section 4b is explicit that it "...does not require the Collective Work apart from the Work itself to be made subject to the terms of this License."

If you do modify the image (eg by adding a new element to a chart, making a composite of two photos, etc), then you have to release that new image as CC-BY-SA, but you can still incorporate it into your non-SA book as a single collective-work element.

Having said all that, 1a could certainly do with some broader examples of what constitutes a collective work, to avoid ambiguity...



Edit (October 2015): a US district court has recently discussed a similar issue (a CC-BY-SA image being used as the cover of a book) and held that that situation clearly constitutes a "collective work":


  Because this 112-page book of maps is not in any way "based upon"” the Photograph, and because defendant did not "recast, transform[], or adapt[]" the Photograph when it used it as the cover art for the Atlas, see License § 1(b), the Court finds that neither the Atlas nor its cover constitutes a derivative work subject to the ShareAlike requirement. Rather, the Atlas is more akin to a collective work, because the Photograph was placed "in its entirety in unmodified form" alongside "other contributions, constituting separate and independent works" – that is, the maps."