how do I get photos from old iMac into iCloud?

Do the following;

1 - download and run the iPhoto Library Upgrader 1.1. on the library on the old iMac.

2 - copy the library to the your current Mac or to an external drive that is formatted OS X Extended (journaled) with Ownership set to be ignored.

3 - open the old library with the Photos app and convert it to a new Photos library.

Now comes the tricky part.

A - Do you current have iCloud Library enabled on your MBA? Also do you have "Optimize Mac Storage" enabled?

B - do you have enough room on your MBA to hold both libraries with full sized originals?

C - how big is the old library, i.e. both no. of photos and file size ?

If you have enough room to add the photos from the old library to your current library I'd make the transfer as follows:

4 - purchase and run PowerPhotos to copy the photos, both edited and original, all metadata, albums and smart albums from the old library into your current library. The only items not copyable are projects and they are copied as albums contains the photos in the project. This PowerPhotos help file explains how: Merging libraries. When done you can delete the old library on the external HD and let the combined library finishing syncing with the iCloud Library.

The other method is as follows:

• Make the old library the System Photo Library with iCloud Library enabled and let it sync with the library in the iCloud.
• When the syncing is done make your current library the System Photos Library with iCloud Library and let it sync with the iCloud library, downloading any new photos from the old library.
• Each of the above syncing could take a week or longer depending on library sizes, internet speeds and space available.

That's why I believe the PowerPhotos method would be best, quicker and done mostly locally.