Double-check the Alt-Text of the banner images. If they have text, the Alt-Text should include that text.
Select the banner and make sure it's set to "Heading 2" in your Canvas page.
To make it mobile-friendly, add the following to your HTML: style="width: 100%;"
For example, your image tag might look something like this: <img id="140431290" src="$CANVAS_COURSE_REFERENCE$/file_ref/g2daa7d2259579557c632ec241d429819/preview" ... />
You want to change it to this: <img style="width: 100%;" id="140431290" src="$CANVAS_COURSE_REFERENCE$/file_ref/g2daa7d2259579557c632ec241d429819/preview" ... />