Can I use iCloud storage to move my photos from one iMac to another?

No, not really. It would require using the iCloud Library on both Macs, purchasing enough additional storage to handle the library and could take a long time, up to a week, for the transfer to complete.

Do you have an external HD available that is not used for Time Machine? If you do then copy the Photos Library from Mac A to it, connect it to Mac B and transfer that way.

If you don't connect the two Macs via Target Disk Mode  and copy the library from Mac A to Mac B that way.

If you are already using iCloud Photos, you can simply create a new, empty Photos Library on your new Mac and enable iCloud Photos on the new Mac. The library will download from iCloud into the empty library on your Mac. iCloud Photos is perfect, if you are using several Macs and want the Photos libraries to be identical on all your devices. It will sync the libraries between your devices and keep them identical, even if the system versions are different. Will you be using the old iMac and the new iMac alternately? Then it would be very useful to sign up for enough cloud storage to keep your Photos Library in iCloud and in sync across your devices.