With the smart_image option set in your job, Blitline will download your zip file and open it and examine it's contents.

If the zip file has 4 or more images, Blitline will choose the largest 4 images and collage them into a 2x2 grid for the output image.

If the zip file has less than 4 images, Blitline will choose the largest 1 image, and output that image as the output image.

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Force Single Image
You can force the output to have only 1 image, regardless of image count inside the zip if you set
"src_data" : { "zip_thumbnail_image_count" = 1 }