How can I activate Scribbles to leave comments in an image?

Go to the settings of your gallery. There you will find the option to activate scribbles in the following location:

Edit Mode ➔ Comments ➔ Image annotations

You must first activate the comments so that you can activate Scribbles, as each Scribble needs a comment to be saved. 

After that, Scribbles in this gallery can be used in Edit Mode in the desktop version of picdrop. In the mobile version of picdrop on smartphones and tablets, you can see the comments and Scribbles, but you cannot currently change them or create new ones.