Gtk.Widget.get_scale_factor is broken and always returns 1. This will make sure your Gtk.Image scales automatically based on scale factor and will fallback gracefully on error.

SmartImage.py

#
# Copyright (C) 2017 Jason Gray <jasonlevigray3@gmail.com>
#
# This program is free software: you can redistribute it and/or modify it
# under the terms of the GNU General Public License version 3, as published
# by the Free Software Foundation.
#
# This program is distributed in the hope that it will be useful, but
# WITHOUT ANY WARRANTY; without even the implied warranties of
# MERCHANTABILITY, SATISFACTORY QUALITY, or FITNESS FOR A PARTICULAR
# PURPOSE.  See the GNU General Public License for more details.
#
# You should have received a copy of the GNU General Public License along
# with this program.  If not, see <http://www.gnu.org/licenses/>.
# END LICENSE

import gi
gi.require_version('Gtk', '3.0')
from gi.repository import GLib, GObject, Gio, Gtk, Gdk, GdkPixbuf

SCALE_FACTOR_OF_ONE = 96


class SmartImage(Gtk.Image):
    __gtype_name__ = 'SmartImage'

    def __init__(
        self,
        fallback_icon=None,
        pre_scale_pixel_size=None,
        **kwargs,
    ):

        super().__init__(
            use_fallback=True,
            visible=True,
            **kwargs
        )

        self._fallback_icon = fallback_icon
        self._pre_scale_pixel_size = pre_scale_pixel_size

        if not self.props.icon_name:
            self.props.icon_name = self.props.fallback_icon

        self._cached_pixbuf = None
        self._screen = Gdk.Screen.get_default()
        self._screen.connect('notify::resolution', self._rescale_self)
        self.props.pixel_size = self._get_max_size()

    @GObject.Property(
        type=str,
        default='image-missing',
        nick='fallback-icon prop',
        blurb='the fallback icon to use incase of error.',
        flags=GObject.ParamFlags.READWRITE,
    )
    def fallback_icon(self):
        return self._fallback_icon or 'image-missing'

    @fallback_icon.setter
    def fallback_icon(self, icon_name):
        self._fallback_icon = icon_name

    @GObject.Property(
        type=int,
        default=4,
        nick='pre-scale-pixel-size prop',
        blurb='the desired pre scale pixel size.',
        flags=GObject.ParamFlags.READWRITE,
    )
    def pre_scale_pixel_size(self):
        return self._pre_scale_pixel_size or 4

    @pre_scale_pixel_size.setter
    def pre_scale_pixel_size(self, size):
        self._pre_scale_pixel_size = size

    def _get_max_size(self):
        scale_factor = self._screen.get_resolution() / SCALE_FACTOR_OF_ONE
        return int(scale_factor * self.props.pre_scale_pixel_size)
        

    def set_pixbuf_from_uri(self, uri):
        def on_read_async(source, result, data):
            try:
                GdkPixbuf.Pixbuf.new_from_stream_async(
                    source.read_finish(result),
                    None, 
                    on_from_stream_async,
                    None,
                )
            except GLib.Error:
                self._fallback()

        def on_from_stream_async(source, result, data):
            try:
                self._cached_pixbuf = GdkPixbuf.Pixbuf.new_from_stream_finish(result)
                self._rescale_self()
            except GLib.Error:
                self._fallback()

        if uri:
            Gio.File.new_for_uri(uri).read_async(GLib.PRIORITY_LOW, None, on_read_async, None)
        else:
            self._fallback()

    def _rescale_self(self, *ignore):
        max_size = self._get_max_size()
        self.props.pixel_size = max_size
        if not self._cached_pixbuf:
            return
        h = self._cached_pixbuf.props.height
        w = self._cached_pixbuf.props.width

        new_h = h * max_size // w
        if new_h <= max_size:
            new_w = max_size
        else:
            new_w = w * max_size // h
            new_h = max_size

        pixbuf = self._cached_pixbuf.scale_simple(new_w, new_h, GdkPixbuf.InterpType.BILINEAR)
        self.set_from_pixbuf(pixbuf)

    def _fallback(self):
        self._cached_pixbuf = None
        self.set_from_icon_name(self.props.fallback_icon, Gtk.IconSize.INVALID)

class SmartImageTest(Gtk.Window):
    def __init__(self):
        Gtk.Window.__init__(self, title='Test')
        self.box = Gtk.Box()
        self.add(self.box)
        self.button1 = Gtk.Button.new()
        self.smart_image = SmartImage(
            fallback_icon='audio-x-generic-symbolic',
            pre_scale_pixel_size=48,
            icon_size=Gtk.IconSize.INVALID,
        )
        self.smart_image.set_pixbuf_from_uri('https://avatars3.githubusercontent.com/u/6667703?s=460&v=4')
        self.button1.set_image(self.smart_image)
        self.box.add(self.button1)
        stream = Gio.File.new_for_uri('https://avatars3.githubusercontent.com/u/6667703?s=460&v=4').read(None)
        self.pixbuf = GdkPixbuf.Pixbuf.new_from_stream_at_scale(stream, 48, 48, False, None)
        self.dumb_image = Gtk.Image.new_from_pixbuf(self.pixbuf)
        self.button2 = Gtk.Button.new()
        self.button2.set_image(self.dumb_image)
        self.box.add(self.button2)
        self.button3 = Gtk.Button.new_from_icon_name('audio-x-generic-symbolic', Gtk.IconSize.DIALOG)
        self.box.add(self.button3)
        self.smart_image2 = SmartImage(
            fallback_icon='audio-x-generic-symbolic',
            pre_scale_pixel_size=48,
            icon_size=Gtk.IconSize.INVALID,
        )
        self.button4 = Gtk.Button.new()
        self.button4.set_image(self.smart_image2)
        self.box.add(self.button4)
        self.set_default_size(-1, -1)
        self.set_resizable(False)
        self.set_border_width(36)

if __name__ == '__main__':
    win = SmartImageTest()
    win.connect('delete-event', Gtk.main_quit)
    win.show_all()
    Gtk.main()