Appreciation for xuzhengyi1995's technical support.
I used a script to catch the pictures from the e-book I bought on BOOK☆WALKER, so the pics' quality looks shitty (I tried 6 times but each time I just got the same file size, so I suspect that the low quality was caused by Bookwalker. But when I watch the book online, the pics look fancier). If anyone has some better plans to achieve pictures in higher resolution from those e-book stores, please teach me!