I was having the same problem as you. The Image was showing up in the email, but it was noticeably worse quality than the original image. I'm not sure if this applies to you guys, but this is what fixed it.
Code:
NSData *myData = UIImageJPEGRepresentation(artworkImageView.image, 0.0);
[controller addAttachmentData:myData mimeType:@"image/png" fileName:@"Albumicon.png"];
}
[controller setSubject:@"Test"];
[controller setMessageBody:(NSString*)all isHTML:YES];
[self presentModalViewController:controller animated:YES];
The above code was my original code with the bad quality image, and the 0.0 after the artworkImageview.image is what was causing it. I change the 0.0 to 1.0 and the quality was perfect.