How to Export JavaScript Chart to Excel file (HighCharts)
HighCharts already supports the image exporting functionality via the Exporting Module which is packaged with it. Exporting After getting it you should be able to modify the script to save the image in any way you need. It's certainly not a beginners task and will require lots of tinkering.
If it were me I would modify the code that responds to the export button so that I can activate it with JavaScript and also pass in information so that the PHP file on the back-end could save the picture in the manner you want rather than returning it to the client.