Use Python 2.7, is has more 3rd party libs at the moment. (Edit: see below).
I recommend you using the stdlib module urllib2, it will allow you to comfortably get web resources.
Example:
import urllib2
response = urllib2.urlopen("http://google.de")
page_source = response.read()
For parsing the code, have a look at BeautifulSoup.
BTW: what exactly do you want to do:
Just for background, I need to download a page and replace any img with ones I have
Edit: It's 2014 now, most of the important libraries have been ported, and you should definitely use Python 3 if you can. python-requests is a very nice high-level library which is easier to use than urllib2.