1: The legend table has the granularity of the layer table. So: one entry in the table per layer. If you define an entry in legend_ico and legend_exp only the first line in the attribute table will be used.
2: legend_ico could contain one path like 'pictures/legend_1.jpg' with a local path for your image. You need to create the image either by hand or place the desired image in the folder you determined. The image should be final in means of it should have the size it will have in the table itself. The above situation would require you to place an image in the export folder in the subfolder 'pictures/'.
Another possibility is to place an image on a filehoster and use the web address of it in legend_ico:
legend_exp contains the text for the table section:
Another possibility for your "categorized" style is to make a screenshot of the legend entry in QGIS and copy it in the desired location after export.