Photoshop Scripting as an api?

One risk is that you'd be violating Photoshop EULA doing that (at least if offering it for other people). Also several such services exists and you might want to look into just building something on top of them. Third point/idea is to take some existing image manipulation library like ImageMagick and mapping that with a web/Javascript API (those probably exist too) so you don't have to do whole stack.

If you have a Windows or Mac platform Web Server  those platforms can run Photoshop. A server application may be able to launch Photoshop passing a file. Perhaps an Open event can be set up that process the passed image file. Where filename is use to trigger the processing option needed to be carried out , At the the end that process would create the images file you want and close down exit  Photoshop. Photoshop closing would continue the server processing being performed that launched Photoshop and waited on its completion.