Your iPhone cannot be backed up because there is not enough space on iCloud

I can't remember the exact text of the message, but it's something like that. I have been getting that message for the last month and it puzzles me on how to fix the issue. The only thing I have being backed up is my photos and I don't see any options for choosing a specific timeframe of photos that I want backed up such as the last 30 days etc. It seems the only option is back up every picture you've ever taken, or don't back up anything at all. If these are my only two options, then ofcourse I will continuously have to keep buying more storage as I take more and more photos if I want to continue using iCloud. This does not make sense.

To complicate issues, when I click on manage settings, it tells me that my iPhone backups are 1.7GB and that I have 3.3GB available. So why is it not backing up?