How can you delete pics on your phone (while on computer) before syncing?
Use Image Capture in the Applications folder. Connect your device and choose it from the sidebar, then select the photos you wish to delete and click the  icon at the bottom.

You can switch the view from list to thumbnails at the bottom too and increase the size of the thumbnails using the slider. Use ⇧-click and ⌘-click as usual to select multiple photos for batch deletion.