Photos Synced iCloud

If you have any photos on an iPhone or iPad that were placed onto the iPhone by syncing them from a computer (using Finder or iTunes), they will be deleted from the iPhone or iPad when you turn on iCloud Photos. Those photos should still be on the computer that they were synced from and will not be deleted from the computer.

Also see: Get help with iCloud Photos - Apple Support

One option would be to use an Internet browser on the computer and manually upload them to the Photos app at www.icloud.com

They would then sync with the phone from there.

Another option, if you want photos on your computer synced with iCloud and all of those photos also synced with the phone, would be to turn on iCloud Photos on the computer if it is a Mac, or to use iCloud for Windows if it is a PC. See: Set up and use iCloud Photos on your Windows PC - Apple Support

If the computer is a Mac, another option would be AirDrop. (sorry, just noticed you said it's too old)