Written before 1995 in all likelihood; certainly before 2000. The cover may have depicted a painting of a dragon on a wall, but I am not certain.
The story, as best I can recall, is that there are a boy and a girl; the boy is ill-tempered and a trouble-maker. Very early in the story they visit an old (possibly Chinese) man's house, who has a painting of a dragon on his wall. The boy has a dart with him which he throws at the painting, piercing the dragon's wing. This causes a disturbance of some sort; it turns out that the painting is somehow connected to an actual dragon living in the man's well, and now that dragon has a hole in its wing.
In order to make amends, the children are to release the old man's familiar good luck spirit, or something like that, which is trapped in a star which fell to earth some time ago. They have to do some specific sequence of steps - by moonlight, if I recall correctly - but they botch the job somehow and the spirit goes wild and flies away.
They have to pursue it by riding the dragon, which has yet to fully heal; due to its injury, it ends up falling into the ocean and they all wash up on an island somewhere; the dragon, as a result of being exposed to the salt in the ocean, has shrunk down to the size of a lizard and spends some time recuperating in a stream or a river.
My recollection of the story after this point is very vague, but I remember something about them flying through what appear to be clouds but are actually some kind of numbing substance which paralyzes them, and later encountering the spirit, which by this point has calmed down so they can talk to it.