You need to import the image into the ebook first, then display it on a page. Let Sigil do the work for you:
First, import the image. Right-click on "Images" in the book browser (left-hand column) and select "add existing files". Navigate to the image you want, select it, and click OK.
Now go to the html file on which you want to display the image, put the cursor in the place where you want the image to show up, and either click the "insert file" icon on the toolbar, or in the menus choose Insert -> file. In the dialog that results, choose your image file (which you imported above; it should be in the images folder) click OK and voila! There you go.
HTH
Albert
And BTW, since it seems you're new to epubs, may I recommend
Pablo's epub tutorial as a quick and mostly painless way to get started.
Rats, I see Ducks beat me to it again.