This has been answered in comments really but
For (1) for images traditionally you could use
\usepackage[demo]{graphicx}
or simply replace the \includegraphics{...}
by \rule{3cm}{1cm}
in either case, getting a black rectangle, but most modern installations now include the examples from mwe
so you can use
\includegraphics{example-image}
to get an image that people will have locally. It comes with several images of various sizes and shapes, see texdoc mwe
for details.
For (2) just take a screenshot (and possibly trim it using your system tools) and upload. As an image. Until you have the rep to post an image you can use the image upload form but remove the generated !
in the markup, which makes it a link. A Higher rep user will put the !
back.
For (3) use the meta site not the main one:-)
\includegraphics{example-image-a}
!
so it makes a link rather than inline image)\documentclass[convert]{standalone}
is suitable, which makes an png file directly