grails won't display images in subfolder using tag

I have image files in grails project such as:

grails-app/assets/images/admin/img1.jpg

I am trying to display it as:

<g:img dir="images/admin" file="img1.jpg">

In development environment (grails 2.4), it is not showing up. I get 404. However, if I copy the image in: grails-app/assets/images/img1.jpg

and change code like below:

<g:img dir="images" file="img1.jpg">

it works.

Any idea why it is not working?

Plugin I am using:

build ":tomcat:7.0.55"
// plugins for the compile step
    compile ":scaffolding:2.1.2"
    compile ':cache:1.1.8'
    compile ":asset-pipeline:1.9.9"

    // plugins needed at runtime but not for compilation
    runtime ":hibernate4:4.3.6.1" // or ":hibernate:3.6.10.18"
    runtime ":database-migration:1.4.0"
    runtime ":jquery:1.11.1"

Thank you.