https://github.com/ricardojmendez/grails-imagetools/archives/master

cd (new directory)
grails upgrade
grails create-plugin ImageTools
grails package-plugin
dir *.zip (to get the full name of the created plugin)
cd (to your project using the plugin)
grails install-plugin (path to created plugin)

import org.grails.plugins.imagetools.*
// To load an image from a buffer
def imageTool = new ImageTool()
imageTool.load(f.getBytes())
// To load an image from a path
imageTool.load("/path/to/image.jpg")

imageTool.saveOriginal()

imageTool.thumbnail(640)  // With the desired thumbnail size

imageTool.writeResult("smaller.640.jpg", "JPEG")

var arr = imageTool.getBytes("JPEG")

imageTool.swapSource()

imageTool.loadAlpha("alpha.jpg")
imageTool.loadMask("mask.jpg")
imageTool.applyMask()

def imageTool = new ImageTool()
imageTool.load("/path/to/image.jpg")
// Reduces the image size
imageTool.thumbnail(640)
// Saves the result
imageTool.writeResult("image_smaller.jpg", "JPEG")
// Crops it to a square
imageTool.square()
/* 
 * Swaps the resulting image with the main one, so that we continue to
 * operate on the square crop.
 */
imageTool.swapSource()
// Creates a copy of the original for later restore
imageTool.saveOriginal()
/* 
 * Iterate through the thumbnail sizes that we want
 *
 */
[178, 133, 69].each {
    def tempName = "image.${it}.jpg"
    /*
     * Creates and saves the thumbnail. It needs to be temporarily saved
     * and re-loaded before applying the masking operation, because of 
     * something that seems to be a JAI glitch.
     * See the following link for details:
     * http://ricardo.strangevistas.net/jai-and-masking-operations.html
     */
    imageTool.thumbnail(it)
    imageTool.writeResult(tempName, "JPEG")
    imageTool.load(tempName)
    // Loads the alpha and mask, and applies them
    imageTool.loadAlpha("Alpha_${it}.jpg")
    imageTool.loadMask("Mask_${it}.jpg")
	imageTool.applyMask()
	// Finally, save it and restore the original so that we can continue
	// to operate on the unmodified image
    imageTool.writeResult(tempname, "JPEG")
    imageTool.restoreOriginal()
}

Error: Could not find mediaLib accelerator wrapper classes. Continuing in pure Java mode
Occurs in: com.sun.media.jai.mlib.MediaLibAccessor
java.lang.NoClassDefFoundError: com/sun/medialib/mlib/Image
at com.sun.media.jai.mlib.MediaLibAccessor$1.run(MediaLibAccessor.java:248)

// domain
class LogFile {
	String name
	String description
	Date createdDate
	Date lastUpdated
	byte[] myImage
    static constraints = {
    }
}

// controller
    def getImageThumbnail = {
    	    def logFile = LogFile.get(params.id)
    	    assert logFile != null
    	    println "Retrieved image size ${logFile.myImage.length}"
    	    def imageTool = new org.grails.plugins.imagetools.ImageTool()
    	    imageTool.load(logFile.myImage)
    	    imageTool.thumbnail(125)
    	    def thumbImage = imageTool.getBytes("JPEG")
    	    println "Returning thumb size ${thumbImage.length}"
    	    response.contentType = "image/jpeg"
    	    response.contentLength = thumbImage.length
    	    response.outputStream.write(thumbImage)    	    
    }
    def getImage = {
    	    def logFile = LogFile.get(params.id)
    	    println "Displaying image size ${logFile.myImage.length}"
    	    response.contentType = "image/jpeg"
    	    response.contentLength = logFile?.myImage.length
    	    response.outputStream.write(logFile?.myImage)
    }

// gsp
<tr class="prop">
  <td valign="top" class="name"><g:message code="logFile.myImage.label" default="Image" /></td>
    <td>
      <img src="${createLink(action:'getImage', id:logFileInstance?.id)}" />
  </td> 	                           
</tr>

// create some instances in BootStrap with images in grails-app/conf/resources
import org.apache.commons.io.IOUtils
import org.codehaus.groovy.grails.commons.ConfigurationHolder
import grails.util.GrailsUtil
import org.ums.LogFile
    def init = { servletContext ->
    	println "**** BootStrap; GrailsUtil.environment: ${GrailsUtil.environment}"
	if (!LogFile.count()) {
    	    def newLog = new LogFile(name:"Sample1",description:"a desc",createdDate:new Date(), lastUpdated:new Date())
    	    newLog.myImage = getFile("console")
    	    if( !newLog.save(flush:true) ) {
    	    	    newLog.errors.each {
    	    	    	    println it
    	    	    }
    	    }
    	    new LogFile(name:"New log 1",description:"a desc",createdDate:new Date(), lastUpdated:new Date(), myImage:getFile("grails")).save(flush:true)
    	    new LogFile(name:"New log 2",description:"a desc",createdDate:new Date(), lastUpdated:new Date(), myImage:getFile("groovy")).save(flush:true)
    	    new LogFile(name:"New log 3",description:"a desc",createdDate:new Date(), lastUpdated:new Date(), myImage:getFile("logging")).save(flush:true)
    }
    def getFile(String fileName) {
    	    def filePath = "resources/${fileName}.png"
    	    println "Importing ${filePath}"
    	    InputStream is = this.getClass().getResourceAsStream(filePath)
    	    byte[] image = IOUtils.toByteArray(is)
    	    println "File size ${image.size()}"
      	    return image 
    }