Hi Christina and welcome to the Canvas Community!
I am assuming you are an instructor that is uploading an image in your course setting so that it appears on the card on the dashboard. If this is correct, each individual controls how that page looks. You can change the overlay behavior by clicking the dots menu to the right of the Dashboard header and click the "Color Overlay" option. If it is checked, click it and it will remove the color overlay on cards that have an image.
Hope this helps and if this isn't what you are looking for, just ask away and we will do our best to help.
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