Apple's iCloud Photo Library service is currently in beta, and for good reason—it currently only allows you to sync images you've taken on your iOS devices, with a Mac app promised in early 2015.
If you still want to get some of your Mac's iPhoto images into iCloud Photo Library before the Mac client's official release, however, there's a pretty easy new way: Last week, Apple started offering an Upload button on the beta version of iCloud.com.
To access it, you'll first have to visit beta.icloud.com. General reminder: Because this is beta software, there might be glitches or data disappearance; as such, we recommend keeping a backup of your photos offline just in case.
Also note that the beta iCloud uploader only supports the uploading of JPG files at this time; you can't upload PNGs or MOVs through the web. (If you want to get those files into iCloud Photo Library, we have a couple other options for you.)
How to upload images to iCloud Photo Library via the web
Once everything loads, click the Upload button in the upper right corner, next to Select Photos.
Choose JPG images from your hard drive that you'd like to upload.
A processing bar appears at the bottom to upload however many items you've chosen. If you wish to stop the upload, you can tap Stop Upload in the bottom right corner.
And that's all there is to it! Apple's upload script is currently very rudimentary—just a file picker that outlaws uploading anything but JPGs — and as such, you might find it tricky for uploading large amounts of pictures. I managed to get a 940-photo session to upload without problem, but that said, you might want to begin with smaller image amounts, just to be safe.