To do that, take a screenshot of mac using the cmd + 3 or cmd + 4 and then you'll have the screenshot window of mac and then press the cmd + c, this will copy that to the clipboard, then try to upload to the workflow.
Nope, that was an image and using the cmd+c from the desktop doesn't copy the image to the clipboard as an image but the file and that's why it's logo was uploaded there.