Batch Resizer

[Vipershark]

IrfanView could probably do the job.
Go to File -> Batch Conversion. Opens a dialog. In the "Batch Conversion settings" area choose your output format (PNG), click "Options", check the "Save Transparent Color" box. "OK". Check "Use advanced options" underneath the output format selection. Click "Advanced". Check "Resize" and choose your settings - the percentage option, 200% and 200%. "OK". Back in the previous dialog, on the right hand, you can go and "Add" the files you want to be converted. Be sure to check where the output will be placed. And then... "Start Batch".
Somehow like that.

Oh, I forgot about Irfanview. Thanks for reminding me. I need to get it.