If you want to use the pc instead of your phone, Photoscape is a great free software programme. I use it to edit all of my pics, crop, resize, add contrast, watermark etc. It also has some nifty effects like adding flares (starbursts of light).
ETA: completely non nail related, but it's also great for editing beauty shots. I took tons of pics recently on a night out and managed to remove everyone's blemishes, I fixed some under eye shadows and even enhanced lip colour! All the girlies looked gorgeous, which was an achievement as we were stuck out in the rain!