RESTful processing function/resource: GET or POST?

Question

I am creating a RESTful web service and some of the resources are computing or processing functions. For instance, it is possible for a user to scale and convert images through the API by submitting an image and receiving the scaled or converted image back.

According to the RESTful Web Services Cookbook, section 2.5, I should use GET:

Treat the processing function as a resource, and use HTTP GET to fetch a
representation containing the output of the processing function. Use query
parameters to supply inputs to the processing function.

This is clear for cases where the inputs are simple (such as the long/lat coordinates of a point). However, should I follow the same advice for larger inputs such as images? As far as I know it is not possible to send this much data as a query parameter.