Were these pictures not taken with your iPhone? The one thing I can think of is that pictures transferred by cable from a computer to an iPhone are treated as temporary, and they don't transfer to iCloud--they figure you have those on the computer. Could that be it?
Otherwise, You can see this:
If you're missing photos or videos in the Photos app - Apple Support
Basically the above says to look in Recently Deleted and in Hidden. Be sure to check your Library thoroughly-- the pictures may not be where you expect, as dates may have changed. You can use the search field with the date or location. And remember, you can sort the Library by date captured or by date added. Also, your Library view now has filters, and you may not have your filter set to "All Items.”
Unfortunately, if you don't keep your own backups to an external drive, then there really isn't any place else to look.