use AppleScript version "2.4"
use scripting additions
use framework "Foundation"
use framework "AppKit"
use imgLib : script "imageDisplayLib"

property NSUUID : a reference to current application’s NSUUID
property |NSURL| : a reference to current application’s |NSURL|
property NSString : a reference to current application’s NSString
property NSImage : a reference to current application’s NSImage
property NSPNGFileType : a reference to current application’s NSPNGFileType
property NSURLComponents : a reference to current application’s NSURLComponents
property NSBitmapImageRep : a reference to current application’s NSBitmapImageRep
property NSMutableDictionary : a reference to current application’s NSMutableDictionary

tell application "Safari"
　　tell front document
　　　　try
　　　　　　set aURL to URL
　　　　on error
　　　　　　set aURL to "https://www.youtube.com/watch?v=_fmDtIV9vvI"
　　　　end try
　　end tell
end tell

if aURL does not start with "https://www.youtube.com/watch?" then return

set urlDict to parseURLParamsAsDict(aURL) of me
set aParam to urlDict’s valueForKey:"v"
if aParam = missing value then return

set imgURL to "https://i1.ytimg.com/vi/" & (aParam as string) & "/mqdefault.jpg"
set newURL to |NSURL|’s URLWithString:imgURL
set aImg to NSImage’s alloc()’s initWithContentsOfURL:newURL

set imgPath to (POSIX path of (path to pictures folder) & ((aParam as string) & "_") & (NSUUID’s UUID()’s UUIDString()) as string) & ".png"

–Save
saveNSImageAtPathAsPNG(aImg, imgPath) of me

–Display
dispImage(aImg, "YouTube thumbnail") of imgLib

on parseURLParamsAsDict(aURL)
　　set components to NSURLComponents’s alloc()’s initWithString:aURL
　　set qList to (components’s query())’s componentsSeparatedByString:"&"
　　
　　set paramRec to NSMutableDictionary’s dictionary()
　　
　　repeat with i in qList
　　　　set keyAndValues to (i’s componentsSeparatedByString:"=")
　　　　(paramRec’s setObject:(keyAndValues’s objectAtIndex:1) forKey:(keyAndValues’s objectAtIndex:0))
　　end repeat
　　
　　return paramRec
end parseURLParamsAsDict