I use IrfanView on Windows most of the time. It's small so it starts up pretty fast and changing quality is a few short clicks away (most of the time the option

I use IrfanView on Windows most of the time. It's small so it starts up pretty fast and changing quality is a few short clicks away (most of the time the options on the save dialog are enough). I don't even have to upload it to somebody I don't know, it works offline too and I even managed to teach it to my mother (she uses the batch tool now). So it can't be that hard.