Some posts want a picture and you have not got one. That is not a content problem, it is a Tuesday.
You can now search a free photo library from inside the Studio, and from Edit slides on a post you already made. It does not spend a post from your plan — only writing does that.
The detail that makes it work
The suggested searches come from the words on the slide you are working on, not from the post topic.
That sounds like a small thing and it is the entire feature. Search the topic and all five slides get five versions of the same picture — which is the obvious failure of every "add images to your carousel" button ever shipped. It reads as a placeholder, because it is one.
Search each slide's own words and the empty living room, the handover of keys and the kitchen at dusk are three different pictures doing three different jobs. It reads as a choice, because it was one.
Two decisions that will outlast the feature
Photos are copied into your account, not linked. A post that renders beautifully today and breaks in March is worse than a post with no picture. Linking is cheaper for us and worse for you.
The photographer is credited in your caption, automatically, and there is no switch to turn it off. That is part of the licence rather than a preference, and we would rather say so than let you discover it later. Nothing is written onto your image — a credit line burned into a paying customer's branded post is permanent, and the caption meets the same requirement without doing that to you.
Where it fits
The picker sits in the composer when you are making the post, and in Edit slides when you are fixing one you already made. The second one matters more than it sounds: before this, changing a picture meant regenerating, and regenerating meant spending a post to swap an image.
Add a photo when you have not got one walks through it.