Saving my Photos that were synched with iTunes

I recently had our local Apple retailer erase my Mac and install the new OS. Mac was full and couldn't figure out what was filling it up. I have back ups on my Time machine but was trying to set it up as a new machine to minimize space used. I have many photos on my iPhone, which were synched through iTunes to my Mac (pre erasing).

When I go in to iCloud to set up photos on my iPhone it tells me it will delete my 5000+ pics on my phone as they were synched with iTunes.

How do I get the pics on my iPhone to get into the cloud so they are accessible from all devices?

I have upgrade my iCloud for space to ensure there is enough room for this.

Please help. Thanks

Jeff