HI
You will find it here:
Fuji Iguana - Google Search
Scroll down, you will find it.
Here is what I would do:
1) Take a screen grab
2) Take it to a friend's computer (or any other computer but your own) and contact the GettyImage sales dept with it.
3) Make up cock and bull story as to why you need this image; act as a customer wanting to buy this image.
4) Ask if they have any similar images on offer. (In case they knocked off more than one of yours. (If there are more)
5) If indeed they have your image(s) and offer it to you (in this case your friend) let your friend buy at least one.
6) Now you can take over in your own right (under your own name) because now you have a case.
On the other hand it is possible that 500px Marketplace is or was an offshoot of GettyImages and use(d) them as a harvesting platform. It is also possible you have unwittingly agreed for 500px to pass on or sell images to GettyImages or others by signing the 500px terms and conditions without reading every singly word therein. If this is the case you are history, as they say in the classics. I wish you well.
Let me know how you go with this.
Use Digimarc to digitally and invisibly watermark your images. The digital watermark is quite robust and is not easily, if all, sidestepped or tinkered with. Digimarc would only be too eager to take up cases of breach as this would demonstrate their worth to prospective customers. GettyImage and cohorts are usually weary of images which are watermarked and leave them alone.