After I realised that there was a limit to what Blogger (GOOGLE) will allow you to store for free, I went through all of my blog photos and reduced them to (at the largest), 2048 pixels on the longer side. So now, all the photos are under the size limit that counts towards my storage. I then deleted all of the original photos from my Google + album (it looks like picasa is on the way out as I was redirected to Google +) I also changed the backups from my phone so they are all automatically reduced in size when saved to Google +
While I was at it, I watermarked all of my original photos (i.e. most of the photos).
One of my concerns in replacing all of my photos was what would happen to a photo that had been pinned on pinterest. I tested another photo from my blog before I replaced and deleted it, and it worked fine. What pinterest does, is take a copy of the pinned photo, and a reference to it's original location. So as long as my post is still there, the photo will still take you back to the page it was pinned from. Needless to say, I am happy for anyone to pin my photos. What I would not be happy with, is if the link back to my blog is broken, or if my photos are used in any other way inappropriately.
How has this affected my used storage? At the time that I made all these changes, my usage reduced by 0.08 GB. That's more than I thought I would have been using for my blog.
Since I completed all these changes in early May, my usage has however increased again. None of the photos that I have been adding now count towards my limit, but Google have yet again had a play with their plans, and I now have a limit of 15GB (up from my previous 5GB limit), shared between Google Drive, Gmail and Google + photos. Since I tend to keep a lot of emails, most of this usage is taken up from Gmail. If I find that I have more problems with usage in future, I know what I have to tackle next. (Eeeek!)