StW I really think you are placing too much emphasis on the illustrations. Look at this one :
Scale & perspective are simply wrong - the figure of Bilbo is is far too small.
So, forget the illustrations. The facts are
1) originally Lake Town was connected to the shore by a number of bridges which could be thrown/cut down fairly quickly & easily.
2) after the story had been submitted to the publisher Tolkien drew the two pictures I've posted which show a single 'Great Bridge'. The Bridge is, according to the text, very large & broad - certainly large & broad enough for Smaug to cross. Tolkien then changes some references in the text to bring them in line with the drawings - but only to the extent that a single Great Bridge has come to replace a number of smaller bridges, The Bridge on the drawings (as with the drawing of Bilbo above) is not 'to scale'.
3) When Smaug attacks Esgaroth is meant to be cut off ('an island in deep water'). So bridges or Bridge have to be thrown down. Smaug's intent was to enter Lake Town on foot but he couldn't do that & had to resort to an aerial attack, making him vulnerable (Tolkien is clear that his only point of vulnerability is the unprotected spot on his left breast - not his eyes).
4) Tolkien changes from bridges to Bridge & thereby creates a problem for himself in that while its possible to cut the bridges its not possible to cut the Bridge. Tolkien ignores that problem & just states that the bridge is thrown down - & if you insist on playing up the importance of the pictures as evidence (though of what I'm not sure), if you take a look at the painting I posted earlier
http://forum.barrowdowns.com/showpos...1&postcount=91 you'll see quite clearly that (however it was done) the Bridge is very definitely thrown down.
Smaug has to be flying to be killed because his ONLY vulnerable point is ONLY accessible from underneath. Smaug either knows this (maybe he felt a draft on that bit when he moved about), or his preferred method of assault is on foot. It doesn't matter. What we know is he intended to use the Bridge & was foiled when he found it wasn't there & so he was forced into making an aerial assault, & that proved his undoing. Pride cometh before a fall, & all that.....
EDIT You might also want to consider the size of the Black Arrow in the painting to the size of Smaug - the arrow is WAY too big if the painting is to scale...