The girl died before she could decide which pair of arms looked best, and Huck thinks the woman in the picture looks “spidery” because she has so many arms.
At this point in the story, Huck is in the home of the Grangerfords, the family that is feuding with the Shepherdsons. He sees the poems and art of daughter Emmeline, who seemed to be obsessed with death, and is now dead herself.
Huck is disturbed by a picture of a girl with too many arms. She had “two arms folded across her breast, and two arms stretched out in front, and two more reaching up towards the moon” (ch 17). Huck realizes that the picture was a draft, and Emmeline was going to decide which pair of arms to keep and erase the rest. Unfortunately, she did not get a chance because she died.
The young woman in the picture had a kind of a nice sweet face, but there was so many arms it made her look too spidery, seemed to me. (ch 17)
This is another example of Huck’s ability to see through the nonsense. He is not impressed by Emmeline’s macabre art or the Shepersons’ grandiosity. He realizes that the family itself is obsessed with death, as a result of their ridiculous duel with the Grangerfords. Twain uses the two families to satirically comment on the futility of feuds.