Extracting entire webpage source code using Selenium

Question

I'm new to Selenium and need help with a task. I want to somehow download the source code of a webpage, change the image src's and then take a screenshot of the resulting webpage. I need to do this using a mobile emulator, hence I am using selenium. The screenshots have to be reflective of the mobile emulator (the screenshots need to be as if you opened the webpage on a mobile device). I know how to open a local html file using selenium as well as take a screenshot through selenium. I also already have the images locally stored in my working directory. However, the page source code that I get from selenium doesn't actually look like if we open that webpage on a mobile broswer. I used the following code to get the source html:

html = driver.page_source

However, if I save this html code and reload it using selenium and then take a screenshot, it looks nothing like the original page (most of the time). The dimensions are ok (that of a mobile browser) but the elements are all missing, even if I haven't replaced the image sources yet. Is there a way to get visually similar code or another way to change the image sources?