def collect_images( images, attribute, url_base ):
    """Iterates over a list of html image elements and returns their corresponding urls as a list of strings.

    Parameters:
        images - A list of BeautifulSoup tags.
        attribute - A string that gives the image element attribute which contains the image's url.
        url_base - A string that gives the base part of the image urls.
    """
    image_src = [None]
    for img in images:
        if img != None and img.has_attr( attribute ):
            image_src.append( url_base + img[attribute] )
    image_src.pop(0)
    return image_src