How do I import a large iPhoto library into an iMac running Yosemite without losing the existing division of the library into albums?which file do I look for on the older Mac, running Snow Leopard?

You don't import one library into another. Simply copy the library from the older Mac to the Pictures folder on iMac. If you already have a library on the iMac rename it first.

Next launch iPhoto on the new iMac with the Option key held down and select the newly copied library.

If you have photos in the original library on the iMac you can merge that library into the larger, copied library with the paid version of iPhoto Library Manager. Or if you also have Aperture 3.3 you can use it to merge the two libraries.

You do NOT import

Never import an iphoto library - it does notd creates a massive mess

you transfer the iPhoto library intact as a single entity - Connect the two Macs together (network, firewire target mode, etc) or use an external hard drive formatted Mac OS extended (journaled) and drag the iPhoto library intact as a single entity from the old Mac to the pictures folder of the new Mac - launch iPhoto on the new mac and it will open the library and convert it as needed and you will be ready move forward. - if your old Mac has iphoto '08 (version 7.x.x) or earlier you must download and RUN the iphoto library upgrader before launching the new iPhoto

LN