Run bash command with current URL as parameter from Firefox and/or safari

I would just do the whole thing at the command line as a shell script instead of using Automator to launch it. Put #!/bin/bash as the first line, and run it as

myscript.sh image*.png

where image*.png is your set of image files that you are apparently selecting in Automator?

If you need to run it on all files in a folder, that can be done too.

You can also test your script in automator using echo ${f%.*} instead of the commands themselves.

This can be done using AppleScript, like in the question you linked to. Watch out, this is not the same as Automator.

Example script:

osascript -e 'tell application (path to frontmost application as text)
set myFile to choose file
POSIX path of myFile
end'

This uses the simplest form of the choose file command, and puts the result in the variable myFile. The result will be of type alias, which has a POSIX path property, which we read in the next line. The result will be written to stdout.

Look here for all the possible optional options to the choose file command. You can e.g. provide a custom prompt text, choose a default location, or only allow certain file types.