Here's a little tip if you're going to upload your audio podcast to YouTube. Size your background image to fill the screen. This will avoid those black borders around your podcast artwork.
- SD 240p: 426 x 240 (px)
- SD 360p: 640 x 360
- SD 480p: 854 x 480
- HD 720p: 1280 x 720
- HD 1080p: 1920 x 1080
- 4K 1440p: 2560 x 1440
- 8K 2160p: 3840 x 2160
480p (854 x 480) is adequate for an audio podcast and would give you a smaller video to upload. Still though, I would recommend 720p (1280 x 720). I know HD might be a bit overkill for an audio podcast and will increase the file size. but your video will look better when played back on TVs. SD videos will often look soft and fuzzy. If you happen to have small-ish text in your artwork, switching to HD would make it easier to read on TV.
Here's an example I posted earlier that used full sized 720p images: Blog Oklahoma Podcast 055: Bold Mission #okvotes18
YouTube also recommends you use a 1280 x 720 sized images for your video thumbnails.
If you don't have access photo editing software like Adobe Photoshop, GIMP, or Paint.net. I can recommend Canva to build your video thumbnails. Canva.com is a free and easy to use online graphics editor. They even have ready made YouTube thumbnail templates for you to use. The image I used for this post was generated from a Canva template.