Google’s My Maps allows the easy creation of pseudo map mash-ups, where you can map your own data as points, lines, and area symbols with Google Maps as the background.
I wrote about My Maps – basic how-to and some of its limits – in another blog post, Allelopathic Maps & Google’s “My Maps.” One of the My Maps limits, the inadequate and corny set of available map symbols, has been removed: you can now create and use your own map symbols in My Maps.
To work with My Maps you need a Google account, and to use custom symbols (icons, as Google calls them) you need some server space to upload your symbols: you must provide a URL to the symbols. You should be able to find a free web hosting service that allows hot-linking (the placement of an image hosted on the free site in a My Maps map in this case). You can also embed images in the pop-up balloon associated with points, lines, and areas on My Maps, and you need server space to host those files.
In Custom Icons for Your Maps, a posting on the official Google Earth/Maps blog, PNG files with transparent backgrounds are recommended, although JPG and GIF should also work. PNGs and GIFs can have transparent backgrounds, essential if you don’t want a white box surrounding your symbol. As Google says PNG, lets PNG.
Making Maps: DIY Cartography 