Hi.
You can avoid webservice altogether, because it is quite complex.
It is very easy to make your own script including a security token and send it data from an external program in XML format.
There should be some identifier in this XML to use the sort by your external program.
What all would need to be done?
1. when importing images, have an identifier in the XML file
2. add a column with this different identifier to the ps_image table
3. override Image.php and add a column with a new identifier to the definition
4. create your own PHP script to update the image position
Alternatively, program a small module that would take care of everything.
It sounds very complicated, but it's not.