Can I upload my photos to iCloud and still keep them on my Mac?

Yes. In fact if you use iCloud Photo Library and delete a photo from the Photos app on your Mac, it will also be deleted from iCloud.

If you open Photos Preferences and click on iCloud at the top of the Preferences window and turn on iCloud Photo Library and also select the "Download originals to this Mac" rather than the "Optimize Mac Storage" option, the full size originals will be maintained on your Mac.

With that many photos it will take quite awhile to upload. Once everything has uploaded, any changes you make to a photo in the Photos app will automatically be saved to iCloud.

Also note that you may need to buy more storage space on iCloud. You can get an idea of how much storage space your photos occupy by using Finder and going to the Pictures folder on your Mac and look at the size of PhotosLibrary. If you need more iCloud storage, the following has information: iCloud storage upgrades and downgrades - Apple Support

You said "iPhoto" -- note that iCloud Photo Library only works with the newer "Photos" app on your Mac.

Classical Bill wrote:

Right. I did mean the new Photos App. Now a couple more questions.

1. I have all my photos organized in albums reflecting where they were taken. Do these Albums transfer to iCloud?

2. I ultimately intend to make slide shows using many, but not all of the pictures taken at a particular location. Can I do this from the photos on iCloud? Or must I create the slide shows prior to uploading?

Thanks so much for your help !!!!

1. Yes

2. Sldeshows don't upload to iCloud Photo Library -- they are only available on the Mac where you create them. See Use Photos and iCloud Photo Library on multiple Mac computers - Apple Support

That seems like a viable alternative -- but note that you can add albums at any time either before or after you initially turn on iCloud Photo Library. Once it is turned on, any new photos added or any changes to albums are automatically synced to iCloud. There will be a period of time before the initial upload completes when you might want to just leave things alone until the upload completes but even that isn't mandatory.