Oops - I just looked at Louisa Farley's profile that you linked and I see that you successfully uploaded a pdf file that contains the image of the death certificate. Good job!
You might be confused by the fact that a pdf file is *NOT* an image file, but is a complete document that can contain both text and image content. WikiTree allows us to upload pdf files the same way as image files, but only displays the icon for a pdf file - you have to click on it in order to see the document. If you copy the image that is in the pdf file, then paste it into a graphic editor in your computer, you can save it from there as a .jpg file. That is an image format and if you upload that file (or a .png or .gif, which are also image formats) then the image itself will be displayed on the profile instead of just the icon for the pdf file.