You can pin posts to your drawings from both the mobile apps and the web app. Depending on the situation you can start a new post while viewing a drawing or start the new post from your Feed and pin it from the create form.
From your mobile device
Option 1: Tap Drawings from the bottom of your mobile screen and select a drawing from the list.
Pan and zoom around the drawing to find the right spot and then long-press to drop your pin. This will launch you into the New Post create form to finish documenting the issue. Once posted you’ll be dropped back into that same drawing.
Option 2: You can also start by creating a new post and then selecting the location/pin icon from the toolbar and selecting Pin to drawing. Once posted you’ll land back in the project Feed.
From the web app
Option 1: Click Drawings from the top of your screen and select the appropriate drawing. Click on the New post on… button from the top right corner, zoom in if necessary and click to pin your post. Once posted you’ll be brought back to the same drawing.
Option 2: Just like on mobile, you can start by clicking New Post from the project Feed. From there click on the "pin to a drawing" icon from the bottom action bar, and then select the appropriate drawing from the dropdown menu on the top left.