Unless a link below states that the site provides free stock photos, with any image you locate and use, credit must be given to the creator with a citation. That's called attribution. See "How to Cite Images" section of this Libguide for how to cite images in different contexts.
"TinyPNG uses smart lossy compression techniques to reduce the file size of your WEBP, JPEG and PNG files."
Looking for icons to spice up your presentation? Check out the Noun Project. It's free and when you download the image, it includes the citation information.
Tip: When you save the icon, you want to be sure to select "Basic Download" in order for it to be free. Save as .png if you want to use it as an image. Save as .svg if you plan to use it in animation.