I was playing with the pageimages API today, in preparation for the Multimedia team's work for next quarter, and I spent about 30 minutes trying to debug PageImages and core, because I overlooked the fine print on the API documentation that explains PageImages will only return one result by default.
As far as I can tell, getting the images is a reasonably performant process, and it won't crash the servers to fetch results for a few more pages in the set. However, I'd be interested to know the rationale for the low default.
We should increase the pilimit by default to 10 or max (50 right now) to avoid the confusing behavior when using the api and only getting an image for the first result on the query.
Update includes/ApiQueryPageImages.php ApiBase::PARAM_DFLT in getAllowedParams() from 1 to 50.