First, revert any changes you made to the theme’s coding, to see if that solves the issue.
Next, go into the block/section’s settings, within the theme editor, and see if there’s an option for the image size, cropping, or formatting.
If you let me know which theme you’re using, and then which block/section you’re using for the banner, then I can try testing on my store to see if I can replicate the issue (and/or solve it for you).