I have the same question. I found that this documentation indicates that you can add " a thumbnail that is displayed when the tool is added to the Analysis gallery." However, I added a thumbnail to several scripts and they still show the default script icon in the gallery. I attempted re-adding them to the gallery after adding the thumbnails and that didn't solve the issue. I also added different types of thumbnails (jpg, png, etc) and that didn't help either. I'd really love for this to work as we are creating these tools for very new GIS users and the tool names need to be longer than can show in the gallery. It'd be nice to have that visual recognition that they are clicking on the right tool and not have them all look the same.
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