I'll have to toss "IrfanView" in as a JPEG/PNG resizer. Pretty easy.
Also, it has a Batch Resize mode that I used a few weeks back to resize several thousand JPEGs used by Tom Willows. He had used ffmpeg to convert a video file to JPEG (20 jpegs per second of video). But, his P10 matrix is larger than mine, so it did not fit.
I used the Batch resize and it converted the thousands (9,019) in about 20 seconds from his resolution to mine.
The quality might have been better if I'd just used ffmpeg on the original, but it looks pretty good.