PS A clarification: I also tried to upload the image at a higher resolution, but the result is the same. The problem is not related to the size of the file.
You could make another project with just that image and then publish a video to quickly see how the image will look in a published video.
If the image still looks grainy in the video, I think there is an "image quality"setting that you can change.
Try a setting of 1600. (I believe the quality setting is a numerical value that sets a limit for the width of any imported images. So if you import an image that is 1552 wide and your quality setting is 800 then the image will be downscaled by videoscribe to 800 pixels.) I'd recommend avoiding values higher than 2000 as higher values can cause performance issues.
-Mike (videoscribe user)