Why do some imported jpegs, psd files show as black rectangles?

Question

I recently scanned a dozen B&W 8x10 prints, plus a color  book dust jacket, into Photoshop, saving all to my desktop variously as jpegs, TIFFs and PSDs, then attempted to import the images to my iPhoto library. Some showed up fine but most were simply black rectangles. With Get Into I could see the file size and the format, but that's it.
One failed import was a 20MB jpeg, so file size might be an issue. But all other other failed imports were in the 2.7MB to 12MB. Suspecting file size may be the problem, I used PS to change the image size from 600dpi to 300 dpi, dropping the size to 5MB or so, and that worked. Question: Is there a size restriction on importing files to the iPhoto library? Any advice much appreciated. Thanks!  John Grissim.
MacMini dual-core Intel, OS 10.6.8, iPhoto 6.0.6, CS3 Photoshop

Answer 1

Black and white photos scanned as grayscale are not supported by iPhoto.  Actually it's the color profile, Dot Gain 20%,  which is used that causes the problem.  CMYK color mode photos are also not supported by iPhoto.
For PSD files if you have an alpha channel that can cause problems in iPhoto. 
With iPhoto size does'nt matter.  I've imported 100 MB files without a problem. In resizing you mighty have changed the profile or something that made it usable. 
Scan all b/w photos as color.  You'll get the same result but they will be iPhoto compatible.
If you want to conver the current b/w scans without rescanning you can try one of the conversions apps available at  Toad's Cellar.
OT