I want to do exactly what is shown in the image. Some know the way to do it?
Instead of having a bitmap, it could be better to just use a mesh, like this:
here I also used a shadless white material, to get the idea.
Another way could also be curves with tapers, probably...
Using real geometry, instead of having a fixed bitmap, allows you to adapt the mesh to different needs, and ensures optimal resolution...
You can use Box Masks in the compositor.
Add two Box Mask nodes, make one of them slightly smaller and set it to subtract from the other box mask. Use the result as the factor input of a Mix node, where you mix the original image with the color you want the edge of the rectangle to be.
You can create the "Graphic" in an application like Photoshop or Gimp with an alpha channel. Save it in a format that supports alpha channels like .PNG. Then import that "Graphic" as a plane into Blender. In Blender you can go to "File" > "Import" select "Images as Planes" and check off "Use Alpha"in the import options.