I wonder how I would get a picture into an oval shape on my
computer?  I want to put it on my menu.

Which/What oval shape?

Free or budget photo editors for Mac:
http://graphicssoft.about.com/cs/imageediting/tp/budgetphotoedm.htm

The simplest way I know to make a picture in an oval shape is as follows

- In PowerPoint (or Keynote) open a presentation with a blank background.
- Insert an oval object, making it large to fill the page.
Choose to edit the object and when prompted for fill choose additional
fill options. In the powerpoint dialog there is an option to choose a
picture tab. Use that and choose the picture you want.
This will then be framed in the oval.
Choose file...Export and save the page as a graphic image.

You may want to use something like firewaorks to edit the image file
as a gif and set the white background to transparent. That may make it
easier to layout on your web page.