I made a new banner, it is under byte size requirements, and is exactly 669x150. The file type is .jpg as instructed. But I tried to upload it and keep getting the same message: Invalid size. Image must be 669 wide by 150 high. I know for a fact my image meets that requirement. I don't know what else to do

Are you sure its 669 pixels x 150 pixels? It may be possible that when you created the document to make your banner, when you chose the size, you picked it out to be measured out by something like centimeters or millimeters instead of pixels.

I figured out what happened, I think. My editor kept resizing the banner image, even though on my files it said it was the correct size.