O formato por defeito da Captura de ecra é o PNG mas, por vezes precisamos de alterar para outros (ex. PDF). Pode-se dar uma visita ao Terminal, mas para simplificar escrevi esta Applescript:

[Abrir script no Script Editor - Mac Only]

set FileExtension to {"pdf", "png", "jpeg", "eps", "tiff", "psd", "gif", "pict", "bmp", "tga", "sgi", "exr", "jg2"}

sortFileExtension(FileExtension)

set grabFormat to (choose from list sortFileExtension with prompt "Escolha o formato:" cancel button name "Cancel" without multiple selections allowed and empty selection allowed)

if grabFormat is not false then
    chooseFormat(grabFormat)
end if

on chooseFormat(grabFormat)
    try
        do shell script "defaults write com.apple.screencapture type" & space & grabFormat
        do shell script "killall SystemUIServer"
        display dialog "Formato da captura de ecra: " & grabFormat buttons {"OK"} with icon 1
    on error
        display dialog "Ocorreu um erro. Tente de novo."
    end try
end chooseFormat

on sortFileExtension(FileExtension)
    set old_delims to AppleScript's text item delimiters
    set AppleScript's text item delimiters to {ASCII character 10}
    set list_string to (FileExtension as string)
    set new_string to do shell script "echo " & quoted form of list_string & " | sort -f"
    set sortFileExtension to (paragraphs of new_string)
    set AppleScript's text item delimiters to old_delims
    return sortFileExtension
end sortFileExtension