Adobe Photoshop costs a lot of money. Also it’s a big and unwieldy program. Here’s one somewhat complicated task that you can do without it!
(Actually, although I’m sure that Photoshop can accomplish the following, I don’t even know how).
The problem: you have a picture like this …

Golly, I wrote half a dissertation and made so many videos about this lake, and I swear, it still comes back to haunt me all the time.
Well, anycase, the problem is, you don’t want that white corner in the top. It’s like that because this is clipped from a NASA satellite image, and the image has that angled edge. How can you easily fill it in?
Instead of using Photoshop, you will use Paint3D. Paint3D came for free installed on my (Windows) laptop. But I think you can download it for free … maybe? As long as you have Windows 10?
Ok, so maybe this isn’t such a universal method after all! But if you manage to get Paint3D on your computer, then this is what you do:
Open it up, it looks like this:
Click Menu >> Browse Files >> open up your image.
Now, here come the fun, beautiful, quick part! First, you want to select the white triangle, so you will …
Click on Magic Select. (It really is magic!)
A blue rim appears around the image, and what you do is, drag the white circles until the box is roughly framing the piece you want to get rid of, to whit, in this case, the white triangle:
Then you press Next to the side.
What happens next is that Paint3D magically figures out, based on colors and texture, exactly what you wanted to select. See, after you roughly frame the white triangle, it understands what you actually wanted to do, and it modifies the blue rectangle rim so as to exactly frame what you were after. Isn’t this what we want? Someone that will understand our feelings without us having to utter them?
So now it looks like this:
Also, just suppose Paint3D didn’t 100% capture what you wanted (which can happen if what you’re after is a little more complicated than a simple white triangle). Use the Add and Remove buttons on the right to refine the selection. Paint3D will keep trying to please you.
Also, do you notice the checkbox next to “Autofill background”. This will be important in a second.
When you have the selection just as you want it, press Done.
Your white triangle now has a bunch of white circles around it, that you can use to twist and rotate it.
But I just want to delete it, so I press Delete on my keyboard, and …
Voila! The corner triangle is now filled. Can you even tell where the insertion comes in?
Now you can save your new image.
And, about that “Autofill background”: just suppose that instead of filling the empty triangle in, you just wanted it get rid of the white and have it remain transparent. Just remember to uncheck “Autofill background” and it will do just that.