PicsArt has a very nifty tool called square fit that adds a enlarged blurred layer of your image in a now squared background
I'm not trained in Graphic Design in any way and I confess at first I thought I should post this question on Ask Ubuntu, then I decided it was a better fit for Super User. I ended up here as I figured I should stop somewhere and this meta post seems to indicate this is indeed the place for a specific questions like this.
I just want to manipulate some photos and have my manipulations displayed to the websites I upload them to. Websites I upload these too though (e.g. Facebook profile pictures) will typically forcibly square my image and then cut off pieces of my manipulations. So, to get around this I've been using PicsArt's square fit tool. After the image is squared I can then do whatever I want to the image safe in the knowledge that it will all be in the final uploaded image.
My question is: How do I "PicsArt Square Fit" an image in GIMP or MS Paint or some other free software? I don't always want to have to take out an android device to do this.
Ideally I would like an answer for both since I use both Windows and Ubuntu.