You should either have initGraph() take (**graph) or return the graph. Since the malloc address of graph is local to initGraph.
Something like:
void initGraph(tGraph **graph, int size)
{
tgraph *temp;
temp = (tGraph*)malloc(sizeof(tGraph*));
temp->cities = (tCity**)malloc(sizeof(tCity*) * size);
temp->size = size;
*graph = temp;
}