Iphone 3G-Update iOS 4.2--LOST MY PICTURES

For photos transferred from your computer via the iTunes sync process which is selected under the Photos tab for your iPhone sync preferences with iTunes, deselect Sync Photos under the Photos tab followed by a sync.
Follow this by re-selecting Sync Photos under the Photos tab followed by a sync to re-transfer the selected photos from your computer to your iPhone.
 MacBook Pro 15" Intel Core 2 Duo/2.53GHz/4GB RAM/500GB HD Mac OS X (10.6.5)  iPod w/Video 30GB,  iPhone 4 32GB

Thanks for your quick reply.
However i am not sure I understand you very well ....
I already tried to sync to transfer the pictures. But my problem is that in iTunes at the very bottom of the screen i see the number of pictures (325 to be precise) but if I select Sync Photos under the Photos tab iTunes asks me which photos i want to add from iPhoto ... and the problem is that i cannot find theses 325 pics anywhere on my computer ....
Do you see what i mean ?
macbook alu Mac OS X (10.5.5) new one

Youre welcome.
Does this involve pictures in your iPhones Camera Roll which is where photos that were captured by your iPhone are stored? If so, photos in the Camera Roll are not included with the iTunes sync process except for being included with your iPhones backup which is updated by iTunes as the first step during the iTunes sync process. Photos in the Camera Roll can and should be imported by your computer as with any other digital camera. With a Mac you can use iPhoto or the Image Capture application for this. When there are photos in the Camera Roll, your iPhone is also detected as a digital camera when connected to your computer.
Or does this involve photos that were transferred from your computer via the iTunes sync process which is selected under the Photos tab for your iPhone sync preferences with iTunes? These photos are stored in your iPhones Photo Library, not in the Camera Roll.
 MacBook Pro 15" Intel Core 2 Duo/2.53GHz/4GB RAM/500GB HD Mac OS X (10.6.5)  iPod w/Video 30GB,  iPhone 4 32GB