On desktop (MacOS and Windows) or on Wire for Web:

In the conversation list:

1. Click the name of the conversation where you'd like to share a picture.
2. Drag and drop the picture.

Or:

1. Click the name of the conversation where you'd like to share a picture.
2. Click Picture  .
3. Select the picture you'd like to share.
4. Click OPEN.

On mobile:

In the conversation list:

1. Tap the name of the conversation where you'd like to share a picture.
2. Swipe left on the gallery, or tap the Camera  to choose a picture. 
3. To take a new picture, tap the Shutter .
4. Tap OK to share your picture.