inspect-images: improve command line options

[Marco Vedovati]

I find the combination --output and--file-name options a bit inconvenient from the user perspective. E.g. depending on the value of --output, one should or shouldn't specify a --file-name.

I'd suggest suppressing the --file-name option and embed in --output both the format and the optional output file name.

E.g. --output file --file-name filename should become --output file=filename