Hope this is ok to recommend, I came across this app last week as every single photo I take is too big to post. I was ending up logging onto my laptop and using paint or the MS photo editing software for this, but I discovered this free android app which simply resizes, nothing else, and you can resize batches of photos in one go with a couple of clicks. It's simply called 'photo and picture resizer' and its avatar/image is a blue square within a white square with a rainbow running though it.
