ChemSketch is a free chemical structure drawing software for students.
Yes, even images belong to someone. If you use someone else's image without citing it, you are committing plagiarism. There are exceptions. For example, you may use a copyright free image without citing it,...but are you sure that the image you found is copyright free? Indiana State University wants to keep you from getting involved in legal troubles by teaching you as much as we can about intellectual property use. The punishments can be expensive. But if you show good faith and try to follow the law, then even if you make a mistake, you can lessen the fines and punishments.
If you are in doubt, cite the image. For an assignment and if you are sure of that the image is copyright free, I would still cite the image SO YOUR PROFESSOR KNOWS THAT IT IS COPYRIGHT FREE. It saves any question of impropriety and it doesn't hurt to state where a copyright-free image came from.
Cite the image using the same citation style you are using for the rest of your presentation or paper. For example, ACS Style. Only a URL is not sufficient. The citation allows your readers or audience to go back to the original source. It allows them to check your work. It allows them to verify facts and information. It allows them to see if the source includes any additional information for their own research project. URLs change when websites change. They are not permanent and, therefore, are often only useful for a limited time. Citation styles have methods to cite images. If your style does not, then cite the image as a section of the source (webpage, book, blog, journal article, whatever).
Snipping Tool is a screen capture tool from Microsoft that comes with the Windows operating system. The Snipping Tool will capture screen shots of anything displayed on your desktop.
ChemDraw is a professional software to draw chemical structures and formulas. Not inexpensive. But you should see what is available for professional chemists. A free version is available for use.
