can't upload photos

I was asked to upload photos to a community group’s blog but can’t seem to figure out how to do this. I get errors whenever I try to add a photo. I am not the owner of the blog, would that make a difference? I have tried making the photos smaller but that doesn’t help either. Do I need to download something to my computer so I can contribute to this blog? I have read through responses to this same question from other people and still don’t understand to do.