To upload Photod connect your phone to your iMac and run either Image Capture it iPhoto on your iMac. Image Capture is in the utilities folder. Those apps will allow you to copy photos from your camera roll to your iMac and then save or delete photos from your camera roll.
You are probably trying to sync using iTunes. This is how you copy photos FROM your iMac to your iPhone. To remove previously sync photos start iTunes on the iMac and connect your iPhone to your computer. Select your iPhone in iTunes then select Photos from the row of tabs that are presented. Under the Photos tab DESELECT sync All Photos or DESELECT the albums you want removed from your iPhone. Then click on Apply or Sync and the photos and albums previously synced will be removed.