First, a PDF is not an image, even though it will show it in an image component. At most I think it will only show the first page. I would recommend reading through the following threads to find a different way to display a PDF in the app.
Second, the size of an image in pixels is not as important. What’s important is the ratio you selected for your cards. It looks like you are using a 3:1 ratio for your cards layout. Just make sure your image is 3 times wider than it is tall. Then it will fit in the placeholder for the card image without cutting anything off.