These days, one of the most powerful content marketing tools you can use is video. It's an even better tool if you can showcase several of them on your YouTube page. Once you start showcasing your content on your channel, you'll want to create custom thumbnails to make each one stand out. You can also use them to create some uniformity and give your channel some eye-pleasing organization.
Anytime you upload a video to YouTube, it will create a set of stills to serve as your thumbnail. This is the image people see before clicking on the content. Once you've verified your account, you'll see a Custom Thumbnail option in the upload/edit page.
There are several free graphic design programs out there, but I like to create YouTube Custom Thumbnails on Canva. They give you some really nice templates that allow you to quickly produce an engaging image. You can choose one and easily swap out images or adjust the text. You can also choose a blank format and build one from scratch.
You can see some of the ones I've created on my YouTube Page.