Add new types of images of papers

Give the user option to select various paper images by displaying them and prompting user to select one of them. The user can also enter src link in google colab terminal for paper background image.

Look at the existing image selection and background handling code in the project, likely in a main script or notebook. Understand how paper images are currently loaded and displayed. The feature requires adding a user interface in the terminal to show options and accept a URL input. Test by running the converter with different paper image sources.

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