Automatically pull image size from image files

[jasper-tms]

Currently there's a value image_shape in config.py that users have to set based on the size of their images. This is suboptimal. The image shape should just be read from the images themselves so users don't have to remember to set this, and so that users changing between working on datasets with different image sizes don't have to change this manually each time they swap what they're working on.