I try to get phpthumbofworking on all my contexts. Since I had no luck with the snippet or the output-filter/-modifier I used the direct link to the phpthumb connector:
connectors/system/phpthumb.php?src=assets/images/MY-IMAGE.jpg&w=600&h=250&zc=1.
Everything works fine, as long as I am logged in to the manager. When I log my self out and take a look at the frontend of my page, the Images are not shown, and if I follow the url I get the following error.
{"success":false,"code":401}
The system connector works only with backend access, since it would be an easy way to create a denial-of-service attack on your host. Image manipulations could use a lot of memory.
You have to use a snippet/output filter solution. I prefer the pthumb extra (https://modx.com/extras/package/pthumb) for this.