Hi Alexander,
Unfortunately inserting images into an already defined custom titleblock is not something that has been implemented in Fusion Drawings so far. The only workaround I can possibly think would require you to use AutoCAD to create a custom title-block .DWG file. In this DWG insert your rendering, move the rendering to where you would want to see it in the modeling works-pace of AutoCAD (might take a few tries to position the rendering), upload this .DWGto Fusion and make a drawing from a design using that custom title-block that has the rendering inside of it. This is quite tedious workaround and would not scale well because you would need a custom title-block with each models rendering already inside of it.
Custom Title-blocks/Title-block Authoring - is one of the bigger things we intend to work on improving, once this work gets implemented into Drawings it should become much easier to insert a pre-generated rendering/image into a drawing your working on.
Here is the Drawings Functionality Roadmap: http://www.autodesk.com/products/fusion-360/blog/2d-drawings-check-roadmap/
Best Regards,
Justin Chase