Allocate an UIImageView with the image and add it in the Appdelegate's Window directly over the tab bar. Make sure you disable the user interaction of the ImageView before adding it on the window.
It will be there over the tab bar without taking any action.
Update:
Get the window from appDelegate,
Appdelegate *appDelegate = ((Appdelegate*)[UIApplication sharedApplication]).delegate;
UIWindow *appDelegateWindow = appDelegate.window;
Then allocate the image view.
UIImageView *imageView = [[UIImageView alloc]initWithImage:[UIImage imageNamed:@"image.png"]];
[imageView sizeToFit];
set the frames for the imageview and add it to the window.
[imageView setFrame:CGRectMake(0,380,50,50)];
[appDelegateWindow addSubView:imageView];