On the picture of the hammer, it does look like an image you quickly "erased" the background color of.
So the line could be added by your image edition tool.
Hard to tell without being able to investigate properly your project.
You should post it.
If you run in to a bug or issue in Construct 3, please post it to the GitHub issue tracker here:
https://github.com/Scirra/Construct-3-bugs/issues