Open Lightbox from external image

I have a repeat with the thumbnails of a gallery and by clicking on each thumbnail the lightbox opens and works regularly
Then I have a cover image, which is not part of the repeat, and I would like the lightbox to open even when clicking on this cover
I tried to add the following code to the cover
dmx-lightbox: images = “lightbox1” and actually opens the lightbox, but the first error image “url not supported”
Can we exclude this and view only the others?