How to allow custom image size or disable custom image size within admin

[mrmadhat]

I have an existing project that I'm converting to Timmy. The site editors have used the custom image size feature when adding posts.

After switching to Timmy these images no longer work and I'm trying to figure out the best solution. Do I have to force the editors to use the predefined image sizes (and if so is there a way to remove the custom image size option from the admin) or is it possible to tell Timmy to generate the image when someone uses a custom size?