Copying photos from one Mac to another
If you want to copy photos from a library on one Mac to a different library on another Mac, you can do that with two basic steps. First, follow the instructions in Accessing an iPhoto library on another Mac so that one of your Macs has access to both libraries. Then, you can perform any copying between those libraries just like any other libraries on your Mac, including copying albums, events, or photos, or merging libraries.
When copying between Macs, here are a couple things to watch out for:
iPhoto library open on the other Mac
An iPhoto library can only be open by one copy of iPhoto at a time, so if iPhoto is still running on the remote Mac when you try copying photos, you will probably receive an error message from iPhoto when you try to open that library, complaining that the library is locked. If this happens, make sure to quit iPhoto on the other Mac before proceeding with copying your photos.
Different versions of iPhoto installed on the two Macs
Ideally, both machines should be running the same version of iPhoto. iPhoto Library Manager will allow you to copy between libraries of different versions, but the library that you're copying to must always match the version of iPhoto that's installed on the Mac you're using iPhoto Library Manager on.
If the destination library is newer than your version of iPhoto, you won't be able to perform the copy, and you might want to consider working on the Mac with the newer version to perform your copying.
If the destination library is older than your version of iPhoto, then iPhoto will prompt you and ask if you want to upgrade the library to the new version of iPhoto. If you tell it to upgrade the library, and your other Mac is still running an older version of iPhoto, then you will not be able to access the upgraded library from that other Mac without installing the newer version of iPhoto itself. So, don't upgrade the library unless you're sure you have the right version of iPhoto installed on all the Macs you want to access that library from.