If you only see this thumbnail:
Then you subsequently changed your Actifit post. Maybe you fixed a bug, maybe something different. However.
As soon as you change something later, your preview picture jumps to this Atifit logo.
You can undo this as follows
Go to https://peakd.com/, log in and use the edit function to access your post.
scroll down until you see the cogwheel on the right side next to the hashtags and click on it. An extended settings menu opens.
scroll down until you get to the pictures and graphics and click on the picture that you would like to have as a preview picture. It is then outlined in colour.
Then on update.
That's all.
If the update button doesn't work, you have to change something small in the text, something, so that the input mask notices that something needs to be updated. And then check the order of the pictures again. It may flip again.
But that's really all.