Command line utility to convert image formats of photos
Specific suggestions:


I generally find that ordering functions by their importance is helpful. Moving get_arguments just above or below main would allow readers to get into the functionality immediately. Others prefer to write methods in call order, but this feels a bit arbitrary (should main be first or last?) and not always achievable, because with branching it's possible to have a cycle of callers.
*nix tools usually support any number of paths as their last arguments, operating on each of them. This would certainly be useful here, and means you could even avoid the whole if/else in main because all inputs should be file paths.
argparse has a file type which you can use.
Don't rely on file extensions to correspond to file formats. I would instead rely on PIL to detect the file type and to throw an exception if it doesn't know it.
Abbreviations like inname make the code harder to read. One way to avoid the urge to shorten everything is to use an IDE, because it'll help you auto-complete everything which is in scope.
In the end this script is a small convenience wrapper around PIL. This is one of those rare occasions when I would suggest replacing it with a small shell script.


General suggestions:


black can automatically format your code to be more idiomatic.
isort can group and sort your imports automatically.
flake8 with a strict complexity limit will give you more hints to write idiomatic Python:

[flake8]
max-complexity = 4
ignore = W503,E203

I would then recommend adding type hints everywhere and validating them using a strict mypy configuration:

[mypy]
check_untyped_defs = true
disallow_untyped_defs = true
ignore_missing_imports = true
no_implicit_optional = true
warn_redundant_casts = true
warn_return_any = true
warn_unused_ignores = true