300x150 is Pretty Standard.
Its Easier If you use a PSD Render.
But You Used a regular Render.
When Placing Your Render, Use The Lasso tool Delete The outer Background Of The Render.
As In your Signature You Would use the lasso tool to delete the White Around your character.
Btw Right Click It And Click feather. Change to 2 px.
But Otherwise. i good Improvement from the nonsense you posted before.