Where do popular websites get their photos from?

I've always wondered where do some of the most popular niche websites out there get their photos from?
An interior design related website for example, there's a popular one, fresh home dot com, where do they get all those nice high quality photos?
What about travel websites? A post about 'the most visited places in europe' for example. Which would contain beautiful images of natural landscapes and cities...

I know there might be some places out there which offer free photos (no copyright), but still, to run a successful website which posts 2-3 times a day, you would have to get an enormous amount of pictures.

There's websites which offer the chance to download like 1000 pics/month for 700$/month plus usually they have their own photographers.
It's minimal the cost of photos even if it were 4k$/month when you think they are doing from 30k to 10milion a month in profit.. (like buzzfeed or upworthy)
Usually they also have some guy messaging the whole day via mail other websites which has right to the picture asking if they could use it if they link back under the image to the site...

So yeah, if you have 700$ a month, go with one of these subscription websites stock photos otherwise if you have more get photographers and mail all the day other websites about using a bunch of their pictures. that's it.

Is istockphoto one of those websites? Or any other stock photos websites?
What about 500px?

Google: Your keyword -> Usage rights -> Labeled for reuse

This has good free images. Read the conditions, if any:
https://commons.m.wikimedia.org/wiki/Main_Page