Does your iPhoto library on your new Mac already contain images? If not,
you simply need to copy the entire iPhoto Library to your new Mac mini.
Depending on your MacOS version and the size of your library you can use the network (Air drop, a shared folder), boot your old Mac in Target Disk mode (Startup key combinations for Intel-based Macs) and mount it as an external drive, or simply copy the library to an external disk and transfer it to the new Mac from this disk.
But if you have to import the images from the old library into an existing iPhoto Library on the new mac, see this thread:
Yes the new mac already has pics in the library, and also now has the old account copied onto it, so I can access the old pics by going on the old account - the problem is that I want to delete that account as I can't get on iCloud since I migrated it.- I have no idea why that is. I will take a look at the i-sync option and see if that works. cheers
Move the iPhoto Library to the "Shared" folder on your Macintosh HD,
check, if you can open it in iPhoto from your new account in the new location, and if you are sure, everything copied over correctly, you can delete the old account.
You may need to make yourself the owner of the moved library again.
That will best be done from the Terminal in your new account:
Open a Terminal window and enter:
chown -R $LOGNAME hit the space bar, then drag your iPhoto Library into the Terminal window, hit "return"
Copy the iPhoto library from the old Mac to the Desktop of the new one. Then download the paid version of iPhoto Library Manager, which is now compatible with Mt. Lion, and use it to merge the two libraries into a new, third library. This will preserve the two libraries in case the merged library isn't to your liking.
Here are screenshots of iPLM's coping and merging preferences:
When you're satisfied that the new, merged libraray is complete and working OK you can delete the two earlier libraries.