If your in windows it should save to a screenshot folder, google that question will probably tell you exact path.
I for one immediately open my paint program (paint.net, free and easy) and then choose "paste-as new image" since windows screenshot also places the pic in your clipboard.
Then you can edit the image to the size you want or make other changes if need be to the picture file.
Many times its easier to get answers if you attach your .capx instead, go to file area of construct2 and choose "save as single file" while in project. Then at bottom of submission window when adding a post you will see area to attach file.