The images for the products is located at :
/product_images/ folder
Exactly :

/product_images = the image with original size
/product_images/thumbnails/150x150 = the resized image to 150x150
/product_images/thumbnails/100x100 = the resized image to 100x100
/product_images/thumbnails/90x90 = the resized image to 90x90
/product_images/thumbnails/70x70 = the resized image to 70x70

If you upload the image using the admin section the image uploading
and resizing to this formats.

If you not happy with the rezulted images quality, possible to upload
the image using ftp to this folders.