The authentication prompt you are seeing is due to your slider image being uploaded to the public_ftp/Content directory. While I'm not completely familiar with the premium WordPress theme you are using, likely the image should be uploaded to public_html/wp-content (possibly even a subdirectory of wp-content). While looking into this for you, I found the following documentation from the marketplace for that theme. They mention a help file that should have been included in the theme package that you downloaded that includes the author's instructions for installing and using the theme.
Please let us know if you have any additional questions or need further assistance.
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