Services 3 File upload, create/return image styles
the image created with the style when you request a page displaying that image_style So you have to call it in code.

The default generation method only creates images when they are requested by a user's browser.

image_style_url  Returns the URL for an image derivative given a style and image path

code sample :

// $file object from file_load($fid) 
image_style_url('style name', $file-&gt;uri)