Open your dashboard:, on the left side menu, click on repositories and select advanced messaging.

Select 'Add New' and give your message a name and press 'Next'.

Here you are able to add an image by selecting 'Select Image' (ability to upload your own images), enter the text you wish to show and select a sound. Confirm once you have made you selections and return to the dashboard.

Select groups from the left hand side and select the group you wish to add the message to. Click the 3 dots on the group and select advanced messaging. 

Here you will be able to select the message you wish and press 'Apply'. This action is almost instantaneous but all devices need to be connected to the network for this to work.