If it's an image on a web page, you can generally just drag and drop it into Google Sites.
it will create something like...
<div style="height:270px"><a href="https://www.google.com/search?q=dickens&tbm=bks&tbs=bkv:r"><img alt="Charles Dickens' 200th Birthday" src="https://www.google.com/logos/2012/dickens-2012-HP.jpg" style="padding-top:32px" title="Charles Dickens' 200th Birthday" height="214" border="0" width="500"></a></div>
</span>
If you have an image file locally (like if you did snagit to screen capture something, or had a local image file), you should upload it to google sites, then display that page and you can then drag and drop it to your wiki page (since it's now a URL).
You'll see something like
<img alt="https://773381768754381720-a-1802744773732722657-s-sites.googlegroups.com/site/infovisspring2012classwork/a5/dr-h-test/yoda.gif?attachauth=ANoY7cqLvyNwwoCDatmbPGQpxqwACzHCBsJgpD3XZd6IjJpjXtw-lrDTd-G37nHmQsXVwblygpZXzQvw7fqX94r2PSuDMp0qpKbv1_uY5w8EjlhSuca2FM9IkU3YGzgNwL9N3KMeq5ipIUrNcJxwMpXsLhMLwzxRQfvi6dyYWLEOIzSjIerCH-LEC7_tE5oqAEMujT3lrDNPH232PUgld7RPqii2nGJMJOQ3MuaBpnwOJUB4ozy-1KY%3D&attredirects=0" src="https://sites.google.com/site/infovisspring2012classwork/a5/dr-h-test/yoda.gif"><br>