Remote datasets: Update filenames of remote dataset images

[yvonnefroehlich]

Description of proposed changes

This PR aims to update the filenames of the remote dataset images.
Some of the images are in png and some are in jpg format.

Related to GMT PR GenericMappingTools/remote-datasets#89
Related to PyGMT PR #2728

Preview: https://pygmt-dev--2745.org.readthedocs.build/en/2745/api/index.html#datasets

Reminders

• Run make format and make check to make sure the code follows the style guide.
• Add tests for new features or tests that would have caught the bug that you're fixing.
• Add new public functions/methods/classes to doc/api/index.rst.
• Write detailed docstrings for all functions/methods.
• If wrapping a new module, open a 'Wrap new GMT module' issue and submit reasonably-sized PRs.
• If adding new functionality, add an example to docstrings or tutorials.
• Use underscores (not hyphens) in names of Python files and directories.

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