First parameter of get_resized_image_from_existing_file() is the filename (including path) and not an url. What's the point even with trying to resize an image that's in document root? Seems much easier to resize the image locally instead. And if you are trying to implement some code for resozing uploaded images these image files won't end in the document root anyway - if you can even save them there in the first place as the document root should have write permissions for the webserver for sure!

@iionly Thanks. Yes I am using the full path as follows

$file->->getLargeHeaderOnFilestore();

which displays the path some thing like C:/wamp64/www/data/2015/02/18/44/profile/img_133.jpg , but the file is not resizing. I just want to take the image file from data folder and need to resize it and store it. could you help me ?

You might want to read http://learn.elgg.org/en/stable/admin/plugins.html. And you can also look at the other sections in the docs to find other topics that might help you to get over the initial helplessness of not knowing how to do things with Elgg.

If you have other questions not answered in the docs (or not already answered in exisiting discussion topics), you can of course ask here on the site.