Adding images can quickly be done after batch-adding your inventory.
You can upload multiple images to create interactive photo galleries when editing or creating your items.
You can quickly add images by:
Clicking the "📷" icon for the item from your inventory page. Then, drag and drop images or click "Add Images" to browse your files for an image
Clicking to edit an item and then drag and drop images or click "Add Images" to browse your files for an image
Clicking to edit an item, select the "Details" tab, and then drag and drop images or click "Add Images" to browse your files for an image
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.