Image Resizer is a great one, in my experience, and one that I've been trying out recently. You can resize the images by a percentage of the original, or a max number of pixels, and you can even specify a target filesize, which is really nice for images here. For example, a photo that I uploaded today, I resized by 50% and set the max filesize to 400KB. Other resizing tools online that I've tried only allow a image width or height to be the determining factor, but on Image Resizer, you actually can keep your image large while reducing the filesize and keeping a nice amount of image clarity.