@Suzanneking ...
The color overlay on course cards displayed on your Canvas Dashboard can be removed by following the steps listed in the following Guide:
Look at the section called "Change Dashboard View" and look for the option for "Color Overlay".
Also, @sondraf ... only instructors are able to change the image/picture on course cards displayed on the Dashboard. This is not a feature that students are able to change on their own. Students can, however, change the color of the overlay if they want that displayed on a course card.
I hope this helps to clarify things for everyone.
