That's wise, as an FYI for those following -- the media queries which factor in screen resolution really are more about responsiveness (as you noted) than mobile detection, in my opinion. For developers who want more accurate mobile detection the user agent string and vendor codes are my preference noting once or twice a year these have to be updated based on market change. This is an approach preferred by some because many mobile phones are extremely high resolution these days and don't forget landscape mode. Some companies in my experience prefer mobile detection and redirection to their very highly customized mobile only site or app, but still use media queries for basic responsiveness just like you do.
NO single approach works for everyone, is the point.
BTW, I favor server side processing and caching of images based on image styles that are set to match and properly scale to the same values used in common media queries. This is far better than dynamic Javascript based DOM image resizing on the fly or creating alternate images MANUALLY or simply allowing the CSS to resize. Many popular CMS's allow use of GD or other graphics libraries to process the images server side and cache them once uploaded as part of the content submission process.
Thanks for your comment!