How to filter tags using classes in Python and BeautifulSoup?

I'm trying to scrape images from a website using the beautifulsoup HTML parser.

Every image on this site has 2 kinds of image tags. One for the thumbnail and the other for the larger sized image, which only shows up when the thumbnail is clicked and expanded. Larger tags contain a class="expanded-image" attribute.

I'm trying to parse through HTML and get the "src" attribute of an extended image that contains the source of the image.

When I try to execute the code, nothing happens. It just says the process is done without scraping any images. However, when I don't try to filter the code and just pass the tag as a parameter, it downloads all the thumbnails.

Here is my code:

import webbrowser, requests, os
from bs4 import BeautifulSoup

def getdata(url):
    r = requests.get(url)
    return r.text

htmldata = getdata('https://boards.4chan.org/a/thread/30814')
soup = BeautifulSoup(htmldata, 'html.parser')

list = []

for i in soup.find_all("img",{"class":"expanded-thumb"}):
    list.append(i['src'].replace("//","https://"))

def download(url, pathname):
    if not os.path.isdir(pathname):
        os.makedirs(pathname)

    filename = os.path.join(pathname, url.split("/")[-1])
    response = requests.get(url, stream=True)

    with open(filename, "wb") as f:
        f.write(response.content)

for a in list:
    download(a,"file")