Welcome!
If I understand you correctly, you want to have the entire map made from one big image rather than individual tiles. Please correct me if I'm wrong.
If that's right, here's how to use it for your map. (Sorry if I get a bit technical; I'll be happy to clarify anything if necessary. I also assume that you're familiar with the basics of using Coldstone already.)
Place the image in a subfolder of your game's "Pictures" folder. Create a new map. Use whatever value you want for the movement tiles, and choose the map dimensions such that it will be the same size as your image. (For example, if you're using a 640x480 image with 32x32 movement tiles, the map dimensions would need to be (640/32)x(480/32) = 20x15 tiles.) The ground tiles can be more or less any size, as you'll not be using them.
Once the map is created, create a new layer and position it behind the "player" layer. (That is, above the player layer in the list, which might be somewhat unintuitive.)
Switch to Layers mode and select your new layer. Navigate through the Tile Set window to locate and select your image. Click in the lower-right corner of your map to place the image in the layer. (The click indicates the lower right corner of the image.) You can position the image more precisely with the movement (crosshairs) tool after it's been placed.
Save your map, create a matching location, and you're ready to go.
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