Adding images can quickly be done after batch adding your inventory.

You can upload multiple images when editing or creating your items to create interactive photo galleries.

You can quickly add images by clicking the item's name in your inventory, then dragging and dropping images or browsing through your computer.

Or, when creating or editing the items, you can click the middle column to upload images.
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As you can imagine, uploading images will take a lot of processing power from your system, so it's not possible to upload while using batch add, but you can quickly upload after you post your batch.