PNG (= Portable Network Graphics) is an option.
You can convert an NSImage
to PNG data with (copied from How to save a NSImage as a new file):
NSImage *image = ...;
NSBitmapImageRep *imgRep = [[image representations] objectAtIndex: 0];
NSData *pngData = [imgRep representationUsingType: NSPNGFileType properties: nil];
and create an UIImage
from PNG data with
UIImage *image = [UIImage imageWithData:pngData];