I'm posting this as community wiki, because I'm borrowing heavily from other answers & comments.
According to Wikipedia
Dumpling is a broad classification for a dish that consists of pieces of dough (made from a variety of starch sources) wrapped around a filling or of dough with no filling. The dough can be based on bread, flour, or potatoes, and may be filled with meat, fish, cheese, vegetables, fruits, or sweets. Dumplings may be prepared using a variety of methods, including baking, boiling, frying, simmering, or steaming, and are found in many world cuisines.
However, from the myriad sections underneath that definition & the myriad comments & answers here, it seems that every country has its own definition.
To someone from the UK, a dumpling is a quite specific culinary delight, made from suet & flour & cooked in the steam coming from the stew it accompanies.
The Southern US has a similar dish, (beef or) chicken & dumplings
but it also has a variant that is more akin to a pasta
As soon as you move away from the 'English-speaking West' then dumplings take on more variety.
To the rest of the world, 'dumplings' become more of a 'container' for interesting foods - presumably originally because it makes them easier to pick up & eat.
The 'west' has these too - they simply don't call them dumplings.
A traditional Cornish Pasty - designed so a Cornish miner could eat it by holding the excess pastry so as not to get dirt on his dinner. The outer crust 'ring' would be discarded afterwards, but the entirety of the crust & filling from the inner section would be eaten.
Or there is the more generic pie, which can pretty much contain anything that will stay still long enough to wrap it in pastry
A Shepherd's pie is a 'special case' as by most other definitions it's not really a pie - it sneaks in because it [usually] has meat in gravy underneath & something akin to a pie-crust made of potato.
You can argue forever over what should actually be under the potato - some insist that because it says 'shepherd' it must be lamb & then define the beef version as 'cottage pie'.
Others don't care what the purists say & call anything made in this style shepherd's pie - I picked a veggie version for this picture [just to be contrary]
So, the "answer to the question" is, yes, they could be called dumplings - but not by all native English speakers.
To sum up my own opinion, as a Brit, this is what I make of the 6 pictures you posted. Note: I have no real knowledge of Chinese food, so my perception will necessarily be skewed by that.
None of the following are dumplings to me...
This, to me, is Gyoza - which I know from Japan
The next two are just 'dim sum' I couldn't identify them to any greater degree [nor would I be certain that broad classification is even correct to someone more au fait with the cuisine.
If someone described these as 'Chinese dumplings' I would probably guess what they mean, but it's not what I would ever call them.
Ravioli - the most generic term is simply "pasta"
This might need more 'specialist' knowledge, but most Brits eat Indian food - lots of it, so these are Samosas. If anything, you'd call it a 'pastry'
This is - almost - a Cornish pasty. Definitely a pasty of some sort.