Probably by the time this is written, the recommendations below will have dramatically changed. The advent of AI art and writing tools is quickly changing the gaming landscape, and as a GM they can take your prep to another level. Here's a lay of the land.
This software can be installed locally on your machine and allows you to render images from thin air, using text prompts. The easiest way to getting this installed is to follow the instructions for the Stable Diffusion web UI by Automatic1111. It may look daunting to install, but if you follow the instructions you'll be able to use a web browser to render images right from your computer.
Keep in mind, Stable Diffusion requires a powerful graphics card to render images speedily. A card with at least 4gb VRAM is required, but 12 to 24gb is ideal.
Midjourney is like Stable Diffusion on steroids, and it's dead simple to use. This service runs as a bot in Discord and lets you render four images per prompt. The advantage of Midjourney is that it uses a proprietary AI model that generates high quality images with very little work on your end (you can give it just about any old prompt and you'll get really cool images). It is believed Midjourney does pre- and post-processing to your text prompt as well as other black magic behind the scenes, which Stable Diffusion can only approximate with extensive prompting/tooling or custom models.
Midjourney charges a fee for unlimited generations. You can use both Stable Diffusion and Midjourney together when rendering art for your games.
This incredible large language model (LLM) chatbot is basically the Star Trek's main computer in a browser.
For a GM, its value comes in its ability to serve as a collaborator in developing your adventure. If you find yourself at a loss for ideas, ChatGPT can summon up random lists, historical details, and adventure seeds instantaneously.