Micki...
Yahoo recently started rolling out a newer version of their email service. This has been in beta for quite a while and I've been using the beta version, with no issues, but I know that they have started rolling the new version out to all members. I bet your pics are being blocked by some of the newer security "features". Apparently, some web-users have still not learned that it's unsafe to click on links in email (when the sender is unknown), and some pics can really be links, so your email client is blocking them until you explicitly allow them.
Is the person sending you the pics in your contact/address book? If not, add them and see if you can see the pics. Also, there is sometimes a link in the header section which will allow you to "trust" the sender, or show images/links. Do you see anything like that?