The following code prints HTML with responsive images support. It's ready to use and defines three levels of resolution: 400px, 800px and original image.

Adapt and start using the following code in Go (we use Picturefill as polyfill):

package main

import "fmt"

func main() {
		imageUrl := "http://images.rethumb.com/image_coimbra_999x999.jpg"

		small_width := "400" // Image width for small resolutions (less than 400px). 
		large_width := "800" // Image width for medium resolutions (less than 800px).
		image_width := "999" // Image original width (used for resolutions larger than 800px).

		fmt.Println(
			"<script src=\"https://cdnjs.cloudflare.com/ajax/libs/picturefill/2.3.1/picturefill.min.js\"></script>\n"+
			"\n"+
			"<img src=\"http://api.rethumb.com/v1/width/"+small_width+"/"+imageUrl+"\"\n"+
			"\n"+
			"     srcset=\"http://api.rethumb.com/v1/width/"+small_width+"/"+imageUrl+" "+small_width+"w,\n"+
			"             http://api.rethumb.com/v1/width/"+large_width+"/"+imageUrl+" "+large_width+"w,\n"+
			"             "+imageUrl+" "+image_width+"w\"\n"+
			"\n"+
			"     sizes=\"100vw\" />")
}

Use the following commands to get started:

$ git clone https://github.com/rethumb/rethumb-go-examples.git
$ cd rethumb-go-examples
$ go build responsive-images.go
$ ./responsive-images