English: Image from postcard titled "Frederick & Nelson Store, Seattle, Washington".
You date a photo by what is and what isn't.
But first, where? This one's a bit disorienting. F&N is now occupied by Nordstrom of course. If you use Google Maps Streetview a bit and look at the building's profile, you'll deduce that this is at the intersection of Westlake and Pine, looking northeast with Capitol Hill in the distance on the far right. It's confusing because now there is no road to our left. That's Westlake (Center) Mall. The satellite map will show you that the Monorail station now sits over the old Westlake roadway.
So, when?
A couple of framing photos will help. Here's the same angle from MOHAI, showing an earlier structure with Frederick and Nelson in the back. They've got circa 1917, since F&N was newly opened. MOHAI has another very helpful photo from 1941. Certainly we knew it was earlier than 1941 already, but look at that hulking building in the distance.
That's the Medical Dental Building. I posted a postcard of it awhile back, when it was new in 1925. My interest was the Arne Sunde street clock. But now it tells us that our F&N photo predates 1925.
Quite a bit of digging in the Seattle Times gives December, 1918 as the opening of the Owl Drug at Westlake and Pine. So we've narrowed it to between 1919 and 1924.
Further digging reveals the name of the building with Owl Drug: the Silverstone Building. And one of the tenants, Rasmussen's, was there "for 10 years" as of 1924. So Rasmussen's was in the predecessor building as well.
I thought I had the date nailed with that clock on Westlake (sorry if you read this the day I posted it). Then I realized two mistakes. 1: the address for the jeweler I had was across the street; 2: the jeweler was there earlier than I thought, because his name was misspelled in the city directory. Hopefully you'll forgive me, because his clock looked exactly like the one here.
Going by what I know about this clock, we're no better off than looking at the buildings and Owl Drug. That clock was installed in December 1919. I'm not sure when it was removed or where it went. It's missing from the postcard of the Medical Dental Building in 1925ish.
The jeweler who owned it was gone by that point, though another jeweler moved in soon after (this was, after all, the north end of the Diamond District). It was present in a photo that ran in the Seattle Times in 1924, and inventoried by the city there in 1924.
Which means that the clock tells us the photo is between 1920 and 1924 (or a bit later).
NOTE: From a set of public domain images collected by Rob Ketcherside. He applied a CC license on Flickr because Flickr does not offer a PD option. The notes are largely his.