icloud photos

My photos have inadvertently been uploaded to icloud and I keep getting messages to "manage my storage". Since I thought the storage issue was on my iphone, I now realize it is referring to my icloud account. My storage is maxed out and I cannot delete or download photos. I want all my photos downloaded to my iMac but I can't figure out how to do this.

If I buy more storage, will I be able to work with these images, i.e delete, download and store on my personal external hard drive; And If I choose to delete my account, will the extra charges stop?