Hello Chris,
Sounds like you either have a broken iPhoto Library or a broken page in iWeb.
Try a test:
Create a brand new Site in iWeb with a blank page and add a picture using the Media browser in iWeb, Media > Photos. Close and re-open iWeb and see if the picture is still there.
If the picture is still there than you have a broken Site or Page and we can discuss that further.
If a new Site does not work then try creating a new iPhoto Library.
1.) Hold down the option key and launch iPhoto (You can use the option key to switch back to your old iPhoto Library when you are done testing.)
2.) Click Create new and save a new library
3.) Add a few pictures to the library
4.) Close iPhoto
Now go back to iWeb and try adding a picture.
If it works then you have a broken iPhoto Library. Switch back to your old iPhoto Library and follow
http://support.apple.com/kb/HT2638 to rebuild your library. Use the last 3 check boxes in the iPhoto Rebuild window.
Hope that helps.