Вот, собственно, она:
const
len=2;
procedure find;
var
searchResult : TSearchRec;
i:integer;
path:string;
a:string;
begin
path:='';
findFirst(path+'*.JPG', faAnyFile, searchResult);
i:=1;
begin
repeat
a:=inttostr(i);
while length(a)<len do
a:='0'+a;
RenameFile(path+searchResult.Name,path+a+'.JPG');
inc(i);
until FindNext(searchResult) <> 0;
FindClose(searchResult);
end;
end;