import bs4

html = """<div class="someClass">
    <a href="href">
        <img alt="some" src="some"/>
    </a>
</div>"""

soup = bs4.BeautifulSoup(html, "html.parser")

# this will return src attrib from img tag that is inside 'a' tag
soup.a.img['src']

>>> 'some'

# if you have more then one 'a' tag
for a in soup.find_all('a'):
    if a.img:
        print(a.img['src'])

>>> 'some'