Transfer Pics from iphone

I need to free up memory on my iPhone 6. I've been using iCloud so all my pics from my iphone 6 are currently in my iCloud library on my iMac. How can I remove my pics on my iPhone without having iCloud erase them as well? I tried manually saving some pics to my iMac computer desktop; eliminate those pics off my iPhone and checking iCloud and they were gone. Then I manually dragged those pics from my desktop into iPhoto and see that they appeared back on my iPhone pictures. Ugg. I don't want to turn off iCloud on my phone, eliminate my photo's and have to leave off iCloud for any future usage because I've read that once I turn iCloud back on, it will assume I wanted all those photo's that no longer appear on my iPhone 'erased' off of iCloud. Any suggestions?