Would it be safe to say picture quality has always been constant?

I look at old photos from the late 19th century and early 20th century such as on Shorpy and when they are blown up, I can't honestly tell the difference in quality between these photos and what my Nikon D5000 can take. Now I'm just an amateur photographer, but to me it seems like photo quality has stayed pretty much constant since photography was invented. Is this true? If not, what accounts for the good quality of old photographs?

Absolutely not. I have never seen any photos from the late 19th century or early 20th that can produce the detail and sharpness of today's cameras. You are obviously not doing something right somewhere.

It would be unsafe to make such an assumption

Your eye is not yet sophisticated enough to see the differences.

Just the major changes in lens design and the precision with how the film went through the more advanced cameras should be easy enough to see. Film may be the deciding factor in the older photos. Film has always been in research and development and the differences between film made in the 1990's is vastly superior to film from the 1960's

What accounts for high quality are two things. The quality of the hardware, film and camera/lenses and the skill of the photographer. No matter how good the camera, unless the photographer has the skills, the images they produce will be nothing more than snap shots

There certainly are very high quality images from the late 19th and early 20th, Shorpy is a great example of such images, here is a link for anybody who does not know the website.

http://www.shorpy.com

Here are just a couple of examples I have grabbed off the site. Dry plate glass 8x10 - 1900-1910.

Another, 1910, dry plate glass.

Zoom in and look at the detail on the bricks.

With some good glass and knowledge of exposure, it's possible to create great image quality no matter the age of equipment or what year it was taken.

I think it's a kind of paradox. Daguerrotypes were amazingly sharp, though they had no positive. Digital images are maybe sharper but they have no negative. Platinum copies from the old times remain (ethernally?). And ink on paper from digital files, who knows.

Picture quality is always improving… Look at some images taken with a d7000 or d3x

You have got to be joking.

Quality of film and sensors has improved in leaps and bounds over the years.

Older cameras took bigger negatives and those negatives needed less enlargement or were even contact printed so one would need a big magnifying glass to find faults like grain or unsharpness. A 5" by 4" negative needs a 4x enlargement to a 20" by 16" print but a 35mm neg' needs a 16x enlargement for the same sized print. This means that if the modern 35mm and the century old 5x4 produced identical prints then modern lenses would have to be 4 times sharper and film would have to be 4 times less grainy. The film certainly did improve that much but lense design has improved but not by that factor. Even now only a Canon EF 135mm 'L' lense beats a 50 year old 135mm Sonnar and that Sonnar's design hadn't changed much in the previous 30 years.