Compress images to upload on website

Compress images and upload them on website using the quality selected by user.

Task: https://openerp.my.openerp.com/web?#id=8680&view_type=form&model=project.task&menu_id=3942&action=327
Pad: https://pad.odoo.com/p/r.BpiW2XLoXNlh35yj

Thanks

Why does an RGB PNG get converted to PNG, but an RGBA PNG get converted to JPEG?