WP Fake Image Replacer generates fake post thumbnail images. Useful in theme development process. Now works with ACF fields.
How can I define thumbnail sizes ?
As usually, juste use the set_post_thumbnail_size() and add_image_size() in functions.php and use the_post_thumbnail('size') in your template file.
How to make it work with ACF ?
Just as usal. Use examples from ACF support site and WP Fake Image Replacer generates fake images for image field and gallery.
What is Developer mode and Presentation mode ?
Developer mode is the default mode during theme developpement process with fakes images (fast and work offline) Presentation mode display random pictures from Flickr, in order to show the website to your clients.