As long as your RSS feed contains properly formatted images, your Hub will automatically pull those images and use them as header images for your Blog Items.
For your Hub, we recommend using images in PNG format. Images should be maximum 1.5 MB, but the higher the resolution and the bigger the image, the better.
On our own blog, we like our header images to take up the width of the blog piece so the dimensions we use are about 840px width by around 400px height.
The tile images that display should be 500px x 272px (or a similar ratio).