function selectall() {
   for(i=0;i<document.selectimages.elements.length;i++) {
      if(document.selectimages.elements[i].type == "checkbox") {
         document.selectimages.elements[i].checked=true;
      }
   }
}

function verifnbchecked() {
   nbchecked=0;
   for(i=0;i<document.selectimages.elements.length;i++) {
      if(document.selectimages.elements[i].type == "checkbox") {
         if(document.selectimages.elements[i].checked==true) {
            nbchecked+=1;
         }
      }
   }
   if(nbchecked>25) {
      window.alert("You have selected more than 25 images.");
   }
}