Our Paint supports importing and exporting PNG images. It does not support interlaced PNG files, if you are unable to import some PNG files, please save that file with other image editing software and make sure to disable interlacing.
Import PNG as a new layer by using File→Import Layer. A dialog box would appear asking you how you would like to interpret the pixel value information in that file. Keeping the options on default should be fine most of the time, however you can also adjust them if the file is saved in a non-standard way.
Using File→Export Image could export your canvas as a composited PNG file. If canvas border is enabled, the composited image would follow the dimension of the cropping. You can also specify color space when exporting the image. It's also suggested to keep it at default value if you don't want to control the color management conversion process.