There are some programs out there on the web that will do this automatically for you. The problem with them is that they choose either all the colors (way more subtle hues than you know what to do with), or they choose the 12 most common colors or so. The latter seems like an OK idea, but in the case of the tropical shot below, the 12 most common colors in the photo are all shades of turquoise. The sand isn't prominent enough to get top billing.
Plus, those programs show you the colors, but don't make for a lovely pin-able color scheme.
Copy and paste your photo onto the appropriate template. Drag the photo layer below the layer labeled "Put photo below this." Re-size and position the photo as needed.
Make sure the check-boxes for "Use All Layers," "Contiguous," and "Anti-aliased" are all checked.