As of today, the keychain's appearance looks slightly different.

Yes, they issued a new update the other day.

Ok, forget keychain, it is not pertinent to this.
In the peakd ui click the pen and click create a post.
Scroll to the bottom, under the hashtags you will see the advanced options button.
Click that and you will see the option to set your preferences.

Let's say you wanted to not use the first picture as the thumbnail, you could change that setting in this advanced feature.
If you wanted to set a beneficiary on a post, that is in here, too.
You can also set your posts as a collection, but I haven't used that functionality and know nothing about how it works.