Hi triplez,
Auctiva's image uploader works best with photos that are a max size of 1280 x 1024 px, and under 1 MB.
If you are uploading larger images (like the 2000 x 2400 ones you mentioned above) then Auctiva's image uploader would automatically resize them, causing a loss in quality.
You will get better images, without a loss in clarity, if you are able to upload images no larger than 1280 x 1024 and still under the 1MB max.
For more info, please see the Image FAQs here:
http://www.auctiva.com/help/faq/images/
Thanks for your question and happy selling!