Great job!

A few suggestions:

1) You may have better results with your pixel art if you change the filteringMode of your SKTextures to SKTextureFilteringMode.NearestNeighbor instead of .Linear.

2) You might want to store SKTextures in your imageDict instead of full SKSpriteNodes. It seems you aren't reusing those nodes, just the textures.

3) Maybe use a plist file to store the level data instead of hard-coded array constants? It's a little more flexible once you abstract the data outside of your source code.