Now that you're accustomed to the fundamentals of designing a GUI application, I'd like to give you some suggestions on how to make ours better.

We can start with the code quality. Do you think this code is good enough, or would you improve it? If so, how? I would test it, and make sure it's robust and caters for all the various scenarios that a user might create by clicking around on the application. I would also make sure the behavior is what I would expect when the website we're scraping is down for any reason.

Another thing that we could improve is the naming. I have prudently named all the components with a leading underscore, both to highlight their somewhat private nature, and to avoid having name clashes with the underlying objects they are linked to. But in retrospect, many of those components could use a better name, so it's really up to you to refactor until you find the form that suits you best. You could start by giving a better name to the _sb function!

For what concerns the user interface, you could try to resize the main application. See what happens? The whole content stays exactly where it is. Empty space is added if you expand, or the whole widgets set disappears gradually if you shrink. This behavior isn't exactly nice, therefore one quick solution could be to make the root window fixed (that is, unable to resize).

Another thing that you could do to improve the application is to add the same functionality we had in the script, to save only PNGs or JPGs. In order to do this, you could place a combo box somewhere, with three values: All, PNGs, JPGs, or something similar. The user should be able to select one of those options before saving the files.

Even better, you could change the declaration of Listbox so that it's possible to select multiple images at the same time, and only the selected ones will be saved. If you manage to do this (it's not as hard as it seems, believe me), then you should consider presenting the Listbox a bit better, maybe providing alternating background colors for the rows.

Another nice thing you could add is a button that opens up a dialog to select a file. The file must be one of the JSON files the application can produce. Once selected, you could run some logic to reconstruct the images from their Base64-encoded version. The logic to do this is very simple, so here's an example:

with open('images.json', 'r') as f:
    data = json.loads(f.read())

for (name, b64val) in data.items():
    with open(name, 'wb') as f:
        f.write(base64.b64decode(b64val))

As you can see, we need to open images.json in read mode, and grab the data dictionary. Once we have it, we can loop through its items, and save each image with the Base64-decoded content. I'll leave it up to you to tie this logic to a button in the application.

Another cool feature that you could add is the ability to open up a preview pane that shows any image you select from Listbox, so that the user can take a peek at the images before deciding to save them.

Finally, one last suggestion for this application is to add a menu. Maybe even a simple menu with File and ? to provide the usual Help or About. Just for fun. Adding menus is not that complicated; you can add text, keyboard shortcuts, images, and so on.