If the company who implemented your solution has done so according to best practices and using a tokenBundle, you should be able to do so from the community branding (paintbrush widget) and simply udpate the image for the branding to be updated.
Otherwise, you will have to replace the component they have created either from the PAtternFormPage Page Settings, and use a default layout or create a new custom theme layout to apply to your page.
Usually, you can find custom theme layouts created by other companies (if not part of a managed package) under Lightning components, via your org settings, if you are not familair with coding, I strongly recommmend agains going and modifying whatever the 3rd party implemented, you should probably talk to whoever implemented the solution to include this option in their custom theme layout.