Currently, you cannot upload multiple pictures directly through the Android mobile app. This is due to a design pattern of Android itself.
As a workaround, you can upload multiple pictures using the Gallery app.
- Open the Gallery app.
- In the Gallery app, open the desired Album and press the menu button.
- Select the checkboxes next to the appropriate pictures.
- Press the Share via button, and then select the Synced Tool app. The selected pictures will upload.
You can also automatically back up your Gallery through the Synced Tool app. For more information, please reference the How Do I Use the Android App Knowledgebase article.