For others experiencing something similar:
The new image resizer in GPP 1.5 tries not to overload your server resources, so it waits until there’s enough resources available to process the images.
If you find it taking too long, you can use a plugin like this: https://en-ca.wordpress.org/plugins/wp-crontrol/
Once installed, check for any cron events with this in the name: image_processing_queue
Manually run the cron event, and your images will resize.
For Bart, this happened automatically, it just took about half an hour due to resource usage. In most cases, it should happen immediately.