This is not meant to be a full list of online image optimizers. There are many other services that are available, but the three described below are the ones that I prefer. Used correctly, these image compressors can greatly decrease the load speed of your website.
TinyPNG (https://tinypng.com) is a basic online image compression service that works well with PNG files. One of its best feature is that it retains the image transparency. It also allows you to batch process up to 20 images, max 5MB each. (Free Service)
Compressnow (http://compressnow.com/) is available in English, French and Spanish. It works with GIF, JPG, JPEG, and PNG images. It did not preserve the background transparency in PNG images and converted the PNG image to JPG. Single processing allows 9MB per image. I used this program as a single processor and received excellent results. (Free Service)
Optimizilla (http://optimizilla.com/) shrinks JPG and PNG images to the minimum possible size while keeping the required level of quality. It allows for image transparency retention. Batch process up to 20 images. (Free Service) You are allowed to click thumbnails in the queue for quality settings and use the slider to control the compression level. I found this optimizer to be excellent for my needs. (Free Service)
The picture that accompanies this post shows before PNG transparent image and after it has been compressed by Optimizilla.