A little bit later on in April (28th or so) Google updated image search. But since your drop-off was two weeks earlier, that alone is probably not your cause. Still, the thread we have about the Image Search update may help to stimulate your thinking:
@Tedster - I cant say that I agree with you but I also do not have the answer.
What I do have is 10 sites that for some reason Image traffic dropped off almost entirely....all between in Mid April...around the 14th. There is another site that dropped its image traffic but that didn't happen until mid May.
All are major sites in their industry...all in different industries.
The challenge here is understanding what changed in the image search algo. And we have precious little understanding of what the algo WAS, so it's very difficult to say what changed. All we know is that some sites that previously enjoyed good traffic from Google Image Search lost that traffic.
If anyone has anything more than that "thin gruel" to share, I'm sure that many people would value it.
Every single site I manage had a drop during this period....So I am with you that it was an algo change. Problem with that though is that in ten years I havent seen a drop this significant so widespread.
One site...20+million visits a month with 5000 per day out of images drop to 35 visits a DAY?
Thought that would help anyone trying to dig through this problem.
I saw this too. In our case, just looking at the logs, the Google image traffic didn't go away, it changed names. If you look carefully in your logs, you'll see it went from one type of referrer (I can't remember exactly, but let's say image.google.com) to something on the main google domain. Both analytics and stat packages like AwStats did not keep up.
I wish I could name names but there are two mashup sites with scraped images ranked top 15 for almost every search since roughly the date you mention. One of the sites scrapes ONLY images and the other much more. Both sites are run by formerly reputable companies and at face value their index pages still look sincere but the scraping is on a monumental scale.
In other words - type in your domain name (ie: example.com just like that) in search and click images... you'll probably find them quickly.
You'll notice a thumbnail of your image is hosted by these two sites with the "view full size" link leading to your original image which is HOTLINKED onto their site and THEY are getting credit for it in all 3 major search engines. The dimensions shown in serps are for your larger version.