Good photos can make or break your click through rates. When people see a photo that intrigues them you have a higher chance of getting them to check out your content. Especially for social media posts, it's vital to have the right picture portray your story.
iStock and Shutterstock are your standard platforms for stock imagery. It's worth getting a subscription if you plan on downloading a lot of photos a month. However, if you're looking for one offs they can get expensive, fast.
Lots of times, it's easier just to look up an image in Google and use it. But when you do that you risk the chance of getting caught with copyright infringement. It's very easy for people to do a reverse look up of the images you used.
Next thing you know, you could be reported for using an image that's not yours. Your account could get deactivated or you can end up with a hefty bill especially if the photo was only meant for editorial use.