Oh! Oh! I know this one!
Adding a custom neighborhood picture
In order to create a custom picture to represent your neighborhood on the main menu, first save the picture you wish to use as a .png file (a 4x3 size ratio works best, but pictures with other proportions will be stretched to fit).
The file must be named N0XX_Neighborhood.png (where XX is the neighborhood number). Then place the file in the neighborhood's directory (e.g., My Documents\EA Games\The Sims 2\Neighborhoods\N004).
(quoted from the TS2 tutoiral, "Creating Custom Neighborhoods for The Sims 2 Using SimCity 4")