Here are the changes you need to make to allow clients on your LAN to ping through the Cisco ASA to a host on Internet/WAN. Basically, this is how to create a class map and a policy that uses the class map. Then, implement the policy.

class-map myclass
match default-inspection-traffic
policy-map mypolicy
class myclass
inspect icmp
service-policy mypolicy interface outside

For a PIX 501:

access-list ping_acl permit icmp any any
access-group ping_acl in interface outside