I use the Sprite Animation Helper plug-in for Paint.net that will cut up a sheet based on cell width and height, and saves each one as a separate image.
It also cycles through the cells you choose (like the animation preview in GDevelop editor). It’ll take a few iterations to get each row of animation, but it’ll save a lot of time and allow you to get the sprite position within the image perfect for animation.
With Paint.net in general, you can also select the background based on colour (and select the threshold so it selects colours close to the selected one) and delete it, making the background transparent.