We got some help on this one via e-mail from former Duluthian Hollie Casteel. 

There was an Alfred T. Dahlquist who was a photographer on East Superior Street around the late 1880s and early 1890s. Is it possible the initials on the photo read A. T. Dahlquist instead of S. F. Dahlquist?  

His address is listed as 17 E. Superior both in the 1891 Duluth City Directory and on the Minnesota Historical Society's Directory of Photographers.  You indicated that the photo says 19 E. Superior St., so there's a bit of discrepancy there.

Assuming the photographer was A. T. Dahlquist, the only Dillner I can find living in the area in the 1880s and 1890s who would be an appropriate age to match the picture, is Swan G. Dillner, born in Sweden in 1861. 

In 1890 he lived in Duluth at SW Grand Forks Avenue, West Park (that's how the address is listed in the 1890-1891 Duluth City Directory). He then is listed as living in Superior on Fisher Ave near 14th, then later at 801 N. 14th in the Superior, Wisconsin City Directories from the late 1890s through 1902. His occupation is listed as a conductor, first for the S R T Ry Co, which I believe stands for Superior Rapid Transit Co., then later with the Duluth S T Co, which I believe is the Duluth-Superior Transit Co. He died in December 1902.

I think it's safe to say Hollie is correct about the initials and the photographer is indeed A. T. Dalhquist. Whether the guy in the photo is Swan G. Dillner or not is more speculative, but a hot lead. Thanks Hollie!