So, I just found out iPhoto doesn't like any format other than JPG.
I just spent the last two hours importing and annotating a large collection of PNGs. I discovered the JPG issue when I noticed a photo at the end of the collection looked a bit "odd". Look closer at the one on the right. I've applied no filters or effects to it. That's as it appears in iPhoto. a 16.7 million colour picture reduced to trash.
GraphicConverter is now batch-converting all my PNGs to JPGs, but needless to say I'm not best pleased at having to repeat the last two hours.
How about a big warning sign on the front of iPhoto saying "this program is braindead and has been hard-wired to only support one graphic format"?
Bug report submitted to rotten Apple.
Update: yeah, yeah, luser error. For some reason, HP's scanning software decided dark photos deserve to be done in 256 colours, not millions. But my problems with manipulating them in iPhoto remain, so I'm still having to redo all of them as JPGs, copying and pasting comments. So far, I've submitted three bug reports to Apple: