Resize Image After Upload (RIAU) is a plugin owned by ShortPixel that does only one but very useful function: resize your images after being uploaded. Say you got a 4000×3000 image: with this plugin you’ll be able to upload it and automatically resize it to the size you want, 1920×1920 for instance.
The plugin also allows to convert any uploaded PNG file to a JPG file, which results in a smaller file. In addition, you can choose the JPEG compression level, overriding WordPress’.
ShortPixel Image Optimizer (SPIO) is ShortPixel’s main plugin, which optimizes your images, new or existing. It has lots of features, and among them, you can also resize the images and convert them to JPG, just as RIAU does. So how would you use both plugins at the same time? The answer is simply that if you are using SPIO, you do not need to use RIAU, as you will not get any benefit from it.