It's tricky.
What solved it for me was:
Screenshot in Base64 of the web;
Write it to disk
Post https://api.telegram.org/botXXXXXXXXXXX:SECRET/sendPhoto
Post using the postdata constructor as example: -- "chat_id",{{chatId}},"photo","file://" + {{Image folder location}} + "screenshot.png"
use multipart content type
