Product import with images easily goes over 4Gb on memory with just couple hundreds of products. The reason for this is that the image model is never properly destoried.
Use a different image processor than Varien_Image.
Add a destruct method into this new processor and register it on shutdown.
Rewrite Mage_Catalog_Helper_Image to change the method validateUploadFile.
Call new processor, call destruct and then return the mime type of the image.