I've used several programs on several platforms to resize images.
For PCs (Windows, Mac, Linux) I usually use GIMP image editor - a bit of a sledgehammer to crack a nut (Its a Photoshop like clone - open source and free.)
On an android phone there are lots of image resizing programs to download for free - the one I used for a while was called, I think, 'Image re-sizer' They usually have adverts that you just ignore.
The built in image viewer on my Chromebook resizes images as as well.
The easiest size to go for/way to go about it that I have found is to crop the image 4:3 for landscape or 3:4 for portrait then resize to 640,480 pixels (landscape) or 480, 640 pixels (portrait). You can post 800, 600 on this forum but not 600, 800 as the max height is 700 so I just stick to 640, 480 both ways round as its easier to get right.
P.S. Its not really fair to call GIMP a clone - its a good tool in its own right and anything that does a similar job to Photoshop is bound to have a similar look and features.