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There have been a few changes over the last few weeks that could affect rankings.  Google released a "Quality Algorithm" that seems to be like Panda but isn't Panda.  More info here: http://searchengineland.com/the-quality-update-google-confirms-changing-how-quality-is-assessed-resulting-in-rankings-shake-up-221118 The mobile friendly algo started to roll out April 22, but I see that you are mobile friendly so that's probably not it. Google also released a new doorway pages algo but we don't know exactly when. If you've got cookie cutter pages (i.e. targeting different cities, etc.) this could possibly be a factor. It's tough to say for certain what the issue is though. I'd also be looking at whether or not you made any changes to the site recently as technical issues could be a factor as well.

Unfortunately, you're both right - using canonical on these could hurt your image search rankings and will remove the individual pages from ranking contention, but at this scale of duplication, indexing all of these is diluting your ranking ability. You could even run into Panda-scale problems or cause enough crawler fatigue that more important pages don't get crawled and indexed. I'd probably lean toward using canonical tags in this case - the ranking ability of your core pages is more important, in most cases, then the ranking ability of the images. This is especially true with Google's new cached image search, where end-users don't even land on your site (they just view the image directly on Google). The other option would be some kind of approach that loads the new pictures in the window but doesn't change the URL. That could make the images themselves crawlable but keep them all on one page. There are a few ways to do this, but they take some engineering.

This is sth that has been happening for a long time, so I would like someone to take a look to the source code of www.hotelelgreco.gr and tell me what's wrong.