Currently, validation of images in structured contents is performed by the ImageValidator class and cannot be changed.
This class makes sure that the image's extension is gif, jpg or jpeg so it is impossible to prevent the upload of gif images for example (or to allow the upload of png files)

There is the same problem with files : it is not currently possible to validate them (to filter pdf or doc files only for example)

Allowing to choose the validator for both files and images would be great !