I’ve got hundreds of stock photos on my computer. They are tagged with people, places, things, emotions, colors, etc. I’ve been collecting them for decades.
You don’t need that many photos to start. Just buy them as you need them. But if you set up your account now, it won’t slow you down later.
I use 123RF for stock photos. They’re very affordable and they have the exact same photos as the expensive services.
When doing PPC or CPM style advertising, it’s a good idea to buy credits in bulk. I usually drop around $100 or so each time my credits run out. The more credits I buy, the less expensive the photos get. At the time of this writing, small photos on 123RF are costing me less than $1.50 each.
Be sure to read the license agreement before using the photos. Basic use includes the right to use the photos in digital format. That includes web use and PDF files.
Licensing larger photos costs more credits. Licensing for print use costs even more. But I find that medium is the largest size that I’ve ever needed.
One last thing to check are the permissions. Some photos are for editorial use only. That means you cannot use them for promotional purposes. Luckily those photos are few in number and they can be blocked from appearing in 123RF’s search results.
As for free stock photos services, I don’t use them. Some of the creative commons licensing is hard to understand, somewhat vague, and they often need special citations, captions, or links. That’s a lot of extra work. There’s also no guarantee that the photographer is authentic and the photo is royalty free.
Besides, there’s a reason why the stock photos are free, which is often a quality issue. If that’s all you can afford then using a service like unsplash.com is better than having no photo, but I’d rather pay two dollars for a high quality, royalty free photo, that I can reuse as long as I want.