Header Logo Only Not Displaying Image – Just a Text Link

I am having trouble getting the image I upload to display in the custom header. I am going into the custom header section of the theme and uploading an image. I then insert the image into the post and nothing happens. It doesn’t appear in the preview. I have selected the header logo only radio button and click save but when I view the updated page it just shows a text link. I do not want to use the standard header image to upload the logo because it is far too large. Thank you for your help!