Just sync your iphone to your computer and open iPhoto. Once it has detected, select the photos you need to import. When they have been imported into an album, you are able to drag them to any destination.Actually you can find more answer on Apple's support website.
If you don't wanna use iPhoto you get a photo backup program to transfer photos from iPhone to Mac.You can easily achive your goal with some simple steps using iPhone Photo to Mac Transfer.Hope it will help
You have to choose either a single folder to sync .
What I did was create a folder named "marks iTunes photos" then drag /drop the photos I want on my Ipad to that folder, then under sync chose that folder to sync.
I find that on the internet How to transfer photo/pictures from iPad to PC,which help me solve my problem.
Hope that help you too.