Why does Gregor cling to the picture of the woman in furs on the wall?

Gregor clings to the picture on the wall because he is clinging to his past.  His sister, Grete, comes in and tries to move all his furniture, thinking it would be easier for him to crawl around the floor and walls if it were gone.  Gregor likes the furniture because it reminds him of when he was normal, but he can't do anything about the fact that his sister and mother are removing it.  In an attempt to do something, he climbs the wall and clings to the picture so his sister cannot remove it.  Ironically, he displays obvious bug-like characteristics in order to cling to his human past.