See my inline.

I have two problems  to ask for your help.

1.I have to calculate  the different date  of list  since it was posted. I
have used calculate field  of which the formular is
=DATEDIF(Created,[today],"d") but it doesn't work.
The error  was
"The security  validation for this page  has timed  out. Click Back in your
Web browser, refresh  the page, and try your operation  again."
--- This message is saying that it took too long to create the field.
Try it again and I bet it will yield a different message. You cannot use
[today] in calculated fields unless today  is a column in the list. After
the formula is saved, you can remove the today column until you need to
edit the formula. It's a quirk of the formula validation.

2. Have you ever customized  a choice  to show  small pictures?
--- Look at using a Data View Web Part to do this.

What should I do sol the problems?