Let me explain how WP Tiles crops and positions its images, so you have an idea of how to get it to work well with your photos.
WP Tiles uses the default WordPress image cropping functionality. When the aspect ratio of a tile is not the aspect ratio of the cropped image, it enlarges the image until the whole tile is covered and then puts it in the center.
By default, WP Tiles (1.0) will grab the 'medium' image size. The user can change this to another size if he wants to. If the image size chosen is set to crop the image, WP Tiles can seem to 'double crop' the image, because it always makes sure the complete tile is covered by the image. WP Tiles does not override this, because it could have actually been the user's intention and because WP Tiles shouldn't generate yet-another-image-size out of the box. So the user has full control over how WP Tiles crops his images - the default is sensible, but not optimal for every website.
Long story short: if you set an image size that is large enough and doesn't crop by default, the image should come out correct most of the time, unless the aspect ratio of the tile is very different and the subject is not centred enough.