Hello Marino! I have some books that I refer to for identification.

Asia Pacific Reef Guide by Helmut Debelius
Nudibranchs & Sea Snails Indo-Pacific Field Guide by Helmut Debelius
Reef Creature Identification Tropical Pacific by Paul Humann & Ned Deloach
& of course there's Google. ^_^

You only need the common group name and a location :) Your example of as you say you know its a crab, you know the location for an example say Bali, my first google image search would be crab species Bali, going down the images I see one that looks very similar, copy and paste the genus name Bali into next search, so on..... Another one to look for is what is the animal perched on when you took the shot, say soft coral, search would be soft coral crab Bali, sometimes works and narrows the search, hope this helps, can take a while to get an ID for someone, does get easier the more you do it, I had to get my eye back in when I first came onto the facebook groups :)

Very detailed guide Stuart, thanks... I'm wondering how you can get to find a scientific name if you don't have the clue of what it is??? Example, it's a crab... and then???

When I ID a species for myself or if someone asks me I follow this routine: Google image search including any key words about the species (ie location, taxonomic group etc). As I find similar looking species I can narrow the search, once I have the genus I can search again with that and most of the time find the species. It is then important to check on one of the main websites that the species is found at the location the photo was took and the species name is still valid. Good accurate websites include: Marlin, Fishbase, Nudibase, Marine.bio, WoRMs (the best one for updated names). Sometimes can be hard working with just photos, some very similar looking species out there, we all get them wrong at times LOL but at least you narrow the search for others to check. Do not forget the genus sp. used in science quite a lot, eg: Triplefin Blenny (Tripterygion sp.) accurate enough for most situations, hope this helps :)