Export png in canvas size, not in image size

Hello Figma Community! I hope you’re all safe and sound!

I need to export images in canvas size instead of image size, but first let me clarify the concept. This conversation is related to banner’s ads, in reality it could be related to everything that has a “frame” with smaller images inside.
Imagine you have a MPU banner, 300x250px (this is what I call canvas size). Inside, the images can either be larger (i.e 600x500px) or smaller (i.e 50x50px). I need to export each one of these images in canvas size, 300x250px, but I cannot figure out how to do it with Figma.

Really appreciate any help.
Thank you!