Hi Andrei,
You can add a message to the login page of your Totara site via Site administration > Plugins > Authentication > Manage authentication. Scroll down to the Instructions box and add in your custom message.
If you want to add an image you'll need to add it as a file on the front page so it can be accessed/viewed by non-logged in users. You can do that on the front page by adding the Main Menu block, adding the image via Add an activity, grabbing the unique url for that image, then (if required) deleting (not hiding!) the Main Menu block. You can then paste that url in when adding the image via the Instructions Add Media button.
If you want to edit the header text 'Is this your first time here?' (shown in my screen shots below as 'Welcome') you can customise the language pack. The stringid is firstname.
Great to hear you're upgrading to v14 as that means you'll be able to add a custom banner on that front page letting folks know all the new features they'll be able to use after that upgrade!
Kind regards,
Meredith