This will vary from client to client and will be based on their wishes, but can really be summed up as follows
The client will provide their own images for use in the project. They will sometimes want control over various other aspects, including palette and copy. In this case, these images remain property of the client.
Client-specified stock images
The client will select certain stock images to use. You will need to negotiate and define who pays for these as part of your bid/contract.
The client will suggest images, and you will need to find appropriate images that meet their requirements. It is likely that you will need to pay for these and factor the cost into your bid/contract. If none are available from stock photo sites, you might need to hire a photographer/designer to produce them for you.
The client gives no direction on what images to use. In this case, you will likely be absorbing the cost yourself and will need to factor this in as part of the bid/contract.
Of course, there are many more possibilities, but this covers the common scenarios. You might also expect overlap between them, as well as different levels of control within a single project (eg they specify a main image but leave the rest to you).
With each situation, you will need to negotiate the image costs with the client, and factor them into your bid if they expect you to foot the bill.