Без обработки исключений и прочих мелочей:
from bs4 import BeautifulSoup
html = """
<div id="main">
<ul class="wookmark-initialised" id="tiles">
<li class="thumbwook"><a class="rel-link" href="/paw/" data-mid="1710"><span class="h2">PAW</span><img src="...." data-src="ссылка1" alt="описание" width="300" height="450"></a>
</li>
<li class="thumbwook"><a class="rel-link" href="/rooppp/" data-mid="1710"><span class="h2">rooppp</span><img src="...." data-src="ссылка2" alt="описание" width="300" height="450"></a>
</li>
<li class="thumbwook"><a class="rel-link" href="/qweqwe/" data-mid="1710"><span class="h2">qweqwe</span><img src="...." data-src="ссылка3" alt="описание" width="300" height="450"></a>
</li>
</ul>
</div>
"""
links = []
bs = BeautifulSoup(html, 'html.parser')
thumbwooks = bs.findAll('li', {'class': 'thumbwook'})
for thumbwook in thumbwooks:
images = thumbwook.findAll('img')
for image in images:
links.append(image['data-src'])
print(links)