Did you convert your iPhoto library to the new Photos app? If so that's why you can't see your photos in iWeb's media browser. iWeb is not compatible with the Photos library. You'll have to export the photos to a folder on the Desktop and add them from there to your iWeb page.
The iPhoto app is still in your Applications folder. If it is any version of iPhoto 9.x you'll be able to obtain iPhoto 9.6.1, which is compatible with 10.10.3, from the App Store by following the directions in this post by Terence Devlin:
Terence Devlin
Apr 14, 2015 11:21 AM
Re: Is Iphoto gone ? i want it back!
in response to Johannes666
Recommended
Go to the App Store and check out the Purchases List. If iPhoto is there then it will be v9.6.1
If it is there, then drag your existing iPhoto app (not the library, just the app) to the trash
Install the App from the App Store.
Sometimes iPhoto is not visible on the Purchases List. it may be hidden. See this article for details on how to unhide it.
http://support.apple.com/kb/HT4928
One question often asked: Will I lose my Photos if I reinstall?
iPhoto the application and the iPhoto Library are two different parts of the iPhoto programme. So, reinstalling the app should not affect the Library. BUT you should always have a back up before doing this kind of work. Always.
This might be a good time to consider migrating to a new website developer aap. One that is actively supported by its developers and is up to date with the latest HTML changes. iWeb was discontinued nearly 3 years ago. It won't be compatible with Yosemite much longer.
One of the apps that many have migrated to is Everweb because it is so like iWeb. There's a demo version available to try. For other potential apps search the iWeb forum for "iWeb alternative" or "iWeb replacement".