Thx for the quick response, kelsos. Today, I tried it again, and what should I say? Now, it takes less than one second to change the picture... So, I don't know, what was the reason for the delay yesterday: slow network, having the app a long time open, large pictures, trying out too many things...? This way is of course no need to change anything!
By the way: Is there a possibility to close the App completely without using a task manager?