Disappearing code…

I have embedded (or keep trying to) a Google map into a page on a site I’m working on. It embeds an image of the map, plus a link to a larger version.

However, every time I go into the page to alter something else, once it’s saved, the image of the map disappears. This means I have to insert the code every time I alter this specific page.

Does anyone know why it’s doing this, please, and how to stop it???!