According to Wikipedia:
Jizō statues are sometimes accompanied by a little pile of stones and pebbles, put there by people in the hope that it would shorten the time children have to suffer in the underworld. (The act is derived from the tradition of building stupas as an act of merit-making.) The statues can sometimes be seen wearing tiny children's clothing or bibs, or with toys, put there by grieving parents to help their lost ones and hoping that Jizō would specially protect them. Sometimes the offerings are put there by parents to thank Jizō for saving their children from a serious illness. Jizō's features are commonly made more babylike to resemble the children he protects.
As he is seen as the saviour of souls who have to suffer in the underworld, his statues are common in cemeteries. He is also believed to be the protective deity of travellers, and roadside statues of Jizō are a common sight in Japan. Firefighters are also believed to be under the protection of Jizō.
So, now you know. There is a story behind rocks wearing aprons.