How to (re)generate image thumbnails

Question

If you want to create a new thumbnail size for your images or if you want to regenerate all existing thumbnails for your images,

Answer

Thumbnails are generated on upload or on image update with such a rule. Here is an example of such a rule, coming from the default module https://github.com/Jahia/default/blob/master/src/main/resources/META-INF/rules.drl

rule "Image update"
    salience 25
    when
        A file content has been modified
             - the mimetype matches image/.*
             - not in operation import
    then
        > long timer = System.currentTimeMillis();
        Add the type jmix:image
        Set the property j:width of the node with the width of the image
        Set the property j:height of the node with the height of the image
        Create an image "thumbnail" of size 150
        Create an image "thumbnail2" of size 350
        Dispose image
        Log "Image " + node.getPath() + " updated in " + (System.currentTimeMillis() - timer) + " ms"
end

So if you want to create a new thumbnail size for your images or if you want to regenerate all existing thumbnails for your images, then you will need to replay this rule for all your images. Here is a useful script that you can play from the JCR console

QueryResult queryResult = null;
Query q = session.getWorkspace().getQueryManager().createQuery("SELECT * FROM [jnt:resource ] AS r WHERE name(r)='jcr:content' and ISDESCENDANTNODE('/sites') ", Query.JCR_SQL2);
queryResult = q.execute();
MultipleNodeIterator iterator = queryResult.getNodes();
while(iterator.hasNext()) {
    JCRNodeWrapper node = (JCRNodeWrapper) iterator.nextNode();
    if (node.getParent().isNodeType("jmix:image")) {
        log.info(node.getPath());
        try {
            def data = node.getProperty("jcr:data").getBinary();
            node.setProperty("jcr:data", data);
        } catch (javax.jcr.PathNotFoundException e) {
            log.info("Could not find jcr:data on " + node.getPath());
        }
        node.save();
    }
}