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The “Before & After” image comparison mode allows you to show two different versions of the same image, and is typically used to show an image in post-processed vs. pre-processed states. The two images occupy the same space, and clicking or tapping on the image will cause it to transition between states.
Proper use of the Before & After mode requires two images, cropped identically. The images should appear in the filmstrip such that the post-processed image is first, and the pre-processed image second:
The post-processed image will appear first on the page, will be used as thumbnail for auto indexing purposes, and will contribute to SEO for the page.
The pre-processed image should have minimal contribution to page SEO, if any at all.
In my own workflow, I typically do not keep pre-processed files for processed images; I only have the processed version(s). The easiest way I find to create a pre-processed version for Before & After comparison is as follows:
Having followed these steps, your pre-processed image rendition should now be cropped identically to the processed version, but without any other adjustments made.