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Thank you all! I tried two methods, and although it was difficult, I managed to do it after listening to both suggestions, even if it's not perfect.
1) I should have thought of copying the empty space from another icon and pasting it where it needs to be (it's not perfect, but it works!).
2) I took a screenshot and created an empty file with a size of 128x128, then inserted the large screenshot into it. Next, I cropped the unnecessary part and resized it (and of course, I didn't forget about the black outline).
Sometimes, by reading other people's answers, you can come up with your own solution...
Another question:
How can I remove the spikes from the champion's shield (the spikes were removed from the horse, but they won't remove from the shield)?