The problem may be a complex one to answer since well only Google really knows whats going on in this situation but from personally experience I would say this is because your images are not unique. Google has the ability to determine if images are unique as well as what colors are in the images using an adopted face recognition algorithm.
It is very common for many websites to use an external domain to host the images such as CDN so that requests are downloaded in parallel so that page rendering is fast, so in turn it makes no difference where your images are hosted because Google indexes them using the master page (I.e you click view original website, and it takes you to the webpage.)
As the same with text content Google does not want duplicates of hundreds of images appearing in the image results after all its not useful for anyone searching seeing hundreds of the same image.
Now the following example of what I believe has happened in this situation is because you have changed URL you have bumped other images to become the new master of the images (If your uploading 3-4 images a day I'm assuming these are not unique and are in fact duplicate).
A client approached me and wanted us to speed up his website - we quickly relised that a CDN would benefit towards a faster site, after 2 months we got a phone call asking why his 2nd site (A site that was never mentioned before now) was ranking on Google images when this had never been the case and his main site was no longer ranking on Google images. We quickly noticed that even though his file names of the images were different they had been completely bumped off.. Even one year on, these images are not in the rankings... Though 'new' pictures are... It's worth noting here, hes a digital photographer so images are all his and unique, fact is of the matter... Google knows what images are what, and I strongly believe that you been bumped off... Do many searches and see if you can spot the same images hosted on another source, this will confirm that.
Not the case
If the images are 100% unique and not found anywhere else on the internet then the chances are you just need to wait a little longer, but using external sources to host images is not BAD SEO since Google has recognized the benefits of hosting media externally.