This depends on where you get your images from.
Certain websites allow the downloading and use of images for a variety of different purposes, including for commercial uses relevant in the context of game development. These licenses can either be for free or require payment of a license fee – the website will clarify the license conditions when you select or attempt to download a picture. Once you have accepted the terms and downloaded the desired pictures, these websites will generate a license agreement between you and the author. It is important that, in each case, you check the terms of the license agreement to determine what the exact parameters of the permitted use are (e.g. if the license specifies “editorial use only”, the image will not be suitable for use in a game). The license agreement may also include other conditions, such as the requirement to attribute the image to the author, meaning that you cannot use the image without simultaneously mentioning the name of the creator.
If you do not download images from a platform providing you with a license agreement, you cannot use that image for game development purposes unless (i) you have otherwise obtained the permission of the author or (ii) the use of that image falls into a specific exception under the applicable copyright law (such as the parodic use of an image in the game). It is recommended to seek professional legal advice to determine whether your use of the image falls under an exception.
The above applies both to static images, as well as to videos.