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Sorry, but you can only add images that are in your media library. This is for both security reasons, and so that HD Quiz can resize and create thumbnails.
You can take a look at the LH Add Media from URL plugin to make it easier/quicker to download images and auto import them to your site – but know that I am not affiliated with that plugin and cannot vouch for how well it will work for you.
You can also consider purchasing the pro version of the FIFU plugin since you are already using it and it seems to have a “Save in the media library” feature – but once again I am not affiliated with that plugin and cannot vouch for it.