There are several ways to get images from Apple's Photos app into a compose window.
How to attach images from Photos in the Compose Window
If you want to attach photos while in the Postbox Compose window, you can access your Photos Events and Albums from the Media Browser.
To do this, open a new message compose window, then click the "Attach" icon Compose Window Toolbar, or the "Image" icon in the Formatting Toolbar.
Within the file picker window, scroll towards the bottom of the left sidebar and select "Photos" under the "Media" heading.
Here you can browse all of your photo content. Simply select the photo(s) you would like to send, and click the "Open" button. You will now find them attached or added to your new message.
Drag and Drop images from Photos App to the Postbox Dock icon
While you are in Apple's Photos App, you can select a group of images and drag and drop them to the Postbox icon in the Dock.
Please note that Postbox must be running.