I'm not a Mac-user, but have read several threads about iPhoto doing things in a very odd way. The "solutions" in those threads point to iPhoto using proxies, and "hiding" the actual photo files.
You might want to do a Search of this forum, via the "Ask a Question - What can we help you with?" [OK, that would be better off it it said something like "Search this Forum."]
I would type in "iPhoto," and then hit Enter/Return, to get a display of threads. From there, start reading, as the solutions are likely in those - if no solutions, at least a lot of good detail on issues.
I'm using Photoshop in Windows. And I think I've been changing some option in photoshop and it become like this....
Well DUH! I could have sworn that I saw reference to iPhoto, but obviously did not.
Please ignore my Reply above, as it is nonsense for Ps on a PC.
I apologize for my total lack of reading comprehension.
Oh, that's okey.. Thanks Hunt ... I think now I have to reinstall Photoshop to fix it if I can't find way to resolve this problem.
Looks like the difference between GPU on (smooth) and off (shows aliasing at non-factor-of-two zoom levels).