The main way to add mathematical graphics to your post is via the upload button. If you want to get super snazzy to generate an interactive graphic, you can do so by embedding a geogebra applet into an iframe.
Note that this question only concerns how to add these things once they’ve been created. The actual creation of images in the first place is outside the scope of this question but you can ask about that kind of thing in the code category. Possibilities include Mathematica, Sage, R, Python, Desmos, and Geogebra.
Image upload
When you’re in the editor, you should notice a toolbar at the top:
Just click on the Upload button as shown. The process should be self-explanatory. You can upload an image from your computer or link to an image on the web.
Dynamic graphs with Geogebra
Geogebra is a great open source tool that allows you to create interactive graphics online. These graphs can be embedded into an iframe on this site, which is how the image below was produced:
The image shows the graph of y=sin(cx) and is interactive. You can move the slider to change the value of c and see the effect.