I am trying to make a single rest api call from a top level site to get results from multiple picture libraries on multiple subsites. Is this possible and if not, what is the best way of approaching this.
I do have a rest api call to retrieve all subsites but I need to retrive results from libraries on a single request.
https://xxxx/_api/web/webs/?$select=title,ServerRelativeUrl"
Unfortunately it is not possible to query multiple lists using a single SharePoint REST API call, as to fetch data from a list you have to specify its parent web and title/ID. Usually for these kinds of queries the best choice is to utilize SharePoint Search REST API (Reference) or, if you can utilize some server side code, you can use the SPSiteDataQuery class to make fetch data from across whole sites or site collections (have a look at this article about SPSiteDataQuery).
Also, have a look at this SO question: Fetch data from multiple list sharepoint REST API in one Ajax call