Photos transferred from your computer via the iTunes sync/transfer process which are photos stored in the Photo Library on your iPhone are optimized for viewing on your iPhone as part of the iTunes sync/transfer process - the original resolution of these photos is reduced. This is why transferring these photos in the opposite direction is not supported.
All photos transferred from your computer to your iPhone should remain on your computer. These photos can be transferred from your old computer to your new computer along with all other important data - documents, etc.
There are some 3rd party paid utilities that provide for transferring these photos in the opposite direction, but the original resolution of these photos will be lost in the process.