ID3 tags are data put inside of your MP3 file before you upload it.
They are NOT needed anymore in the podcasting world due to how the RSS feeds and apps have evolved to use data from the RSS feed over the years. But if you want to use them you can.
Overcast uses the image from the ID3 tag if it’s present, but will use the main image for your show if not.
Do not worry about setting the ID3 tags if you haven’t before. It will work just fine without!