With Bulkly, you can easily use images hosted elsewhere. Most of the time you just need to use the full URL to the image. For example, https://bulk.ly/wp-content/uploads/2020/01/linkedin-showcase-pages-1024×536.png.
However, with Dropbox hosted images you will need to be sure your URLs follow this format.
According to https://help.dropbox.com/
And the image will be like this: