This monogame pipeline extension packs png files into a texture atlas. The packing code was taken from the nuclex framework http://nuclexframework.codeplex.com/

1. Using the content pipeline tool, add Monogame.TextureAtlas.PipelineExtension.dll to your mgcb

2. Create a text file with .atlas extension, this is used to specify a list of target folders containing the pngs. One subfolder per line, currently absolute paths not supported, just use myimagefolder or myimagefolder/myothersubfolder

3. From the pipeline tool, add the .atlas to your mgcb, and configure just like a regular texture. Build

4. In your C# Monogame project add a reference to Monogame.TextureAtlas and load the atlas using the ContentManager:

var myAtlas = Content.Load<TextureAtlas>("myatlas");
//then retrieves the sprites
this.Sprites = myAtlas.Sprites

5. Then using a SpriteBatch extension, draw a sprite:

 SpriteBatch.Draw(Sprites[0], new Vector2(0, 0), Color.White);