I am trying to retrieve images using REST SEARCH API from multiple Picture Libraries distributed across sub-sites(sub-sites are all under a single site collection https://xyz/site/*) .I also need to include various filters, such as 'created by' along with specific call to the sub-site libraries. I am currently retrieving images from the whole site-collection and I not able to limit the call to sub-sites only.
If you want to display results in specific subsites (not all sites in site collection):
In the querytext, enter paths of subsites.
_api/search/query?querytext='path:https://xyz/sites/<subsite1> path:https://xyz/sites/<subsite2> path:https://xyz/sites/<subsite3>'
And you can also add other queries to the querttext to achieve your requirement.
For example, get all pictures created by 'Test' from subsite1 and subsite2.
_api/search/query?querytext='path:https://xyz/sites/<subsite1> path:https://xyz/sites/<subsite2> Author:Test ContentType=Image'