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We all know about SN1987A, the closest observed supernova since Kepler's time. Its progenitor was Sanduleak -69 202, a magnitude 12 blue supergiant, catalogued in 1970. Were there any images of this star before it exploded, and how did we know that this was the star that produced said supernova?

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Yes, it is clearly visible in photographs of the LMC from the 1980s. It is shown in this comparison from the Australian Astronomical Observatory

We know it was this star in the most simple possible way: The supernova occurred exactly at the same location as this star, and when we look now, that star has gone (replaced by a little bipolar supernova remnant)

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Another link worth sharing comes from ESO and is called The Large Magellanic Cloud before and after SN1987A. The link mainly shows the following

enter image description here

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    $\begingroup$ Nice photo. The supernova is clearly visible on the right, but I can't distinguish the progenitor from neighboring stars. Perhaps you could add a (freehand) circle to mark it? $\endgroup$
    – WarpPrime
    Commented May 18, 2021 at 13:23
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Yes. In fact, there were numerous (photographic plate) images of the star, going as far back as 1899(!). From IAU Circular No. 4367:

M. L. Hazen, Center for Astrophysics, writes: "No major change in the brightness of Sk -69 202 was found in a search of the star's area on 502 Harvard blue patrol plates taken 1899-1953 at a focal scale of 600"/mm and on 59 plates of the Damon blue patrol series taken in 1971, 1978-79, and 1981-86 at a focal scale of 580"/mm.

There was also at least one spectrum (Sanduleak 1970), which was the basis for its classification as a B3 I supergiant in Sanduleak's catalog.

Observations of the supernova and retrospective astrometric analysess of the images of established its spatial coincidence with Sanduleak $-$69 207 (e.g., West et al. 1987, Walborn et al. 1987, Girard et al. 1988), which is very suggestive (but not ironclad) evidence that it was the progenitor.

The supernova faded very rapidly in the UV, and so the first post-outburst evidence that Sanduleak $-$69 207 had indeed disappeared came from UV observations by the International Ultraviolet Explorer satellite in March, 1987, which found that the remnant UV flux was consistent with the two neighboring main-sequence B stars only, without any of the expected contribution from Sanduleak $-$69 207 -- which as a B supergiant would have dominated the UV light if it was still there (e.g., Gilmozzi et al. 1987).

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