Thumbnail generation

Lets say that I would have to show a customer how to create thumbnails, in order for him to add it on his news website. Suppose the CMS does not do that for the webmaster.

Thumbnail generation services

So, what services ( online ) or softwares would you recommend me, in order to solve this matter ?

Please list here as much info on that product please, if you would really like to help me.

Thank you