Images as a service is a convenient to handle images upload in a web site.
Here are some features, those types of services are doing.
Image uploader (Optional)
Image resize thanks to an url with special transformations parameters
REST API to manage the stored images
CDN to serve image faster
It allow you to spare the time :
to develop similar functionnality.
to manage only you software code and not the site assets which generally are the heaviest ressources and require specific handling
Here is a comparison table of the different services. It is not easy as the different services use different metrics to price their offers.
Feel free to suggest me other I have not listed here. I included only the first plan below 100$.
Here are also some free alternatives :