Process field before database insert / update
Stripping the junk and such should be done with a custom formfield.
Downloading the images... there are multiple ways to fix that problem.

If you choose to store the image location and original location in the database, than you should do it with a pre-save signal.
If you choose to store the images locally directly, than you can make it part of the formfield aswell. Simply download all remote images and replace the urls with a local url.
I have a Django model with a text field. I'm using a rich text editor (nicEdit) on the admin site to allow the client to easily enter markup into the field. I'd like to process the contents of the field and perform a few actions before anything is inserted into the database.
For example, I want to strip junk generated by MS Word, font tags, etc. I hope this part should be easy, but I'm not sure what to override or hook to get this working.
I also want to detect remotely-linked images, download a local copy to MEDIA_ROOT, and relink the img src to the local image. I'm not quite sure how to go about fetching the remote image; I thought django.Storage might help but it looks like it's unable to fetch content from a remote URL.
Any suggestions?