Just a quick reference 16×9 and 4×3 ratio images.

The 16×9 Ratio:

• 100 x 56 – not ideal (too small, requires image to be stretched)
• 200 x 112 – not ideal (too small, requires image to be stretched)
• 300 x 168 – not ideal (small, requires image to be stretched)
• 400 x 225 – more ideal (small, requires image to be stretched)
• 450 x 253 – most ideal
• 500 x 281 – most ideal
• 550 x 309 – most ideal
• 600 x 337 – most ideal
• 650 x 365 – most ideal
• 700 x 393 – most ideal
• 750 x 421 – most ideal
• 800 x 450 – more ideal (file size)
• 900 x 506 – less ideal (file size)
• 1000 x 562 – less ideal (file size)

The 4×3 (8×6) Ratio:

• 100 x 75 – not ideal (too small, requires image to be stretched)
• 200 x 150 – not ideal (too small, requires image to be stretched)
• 300 x 225 – not ideal (too small, requires image to be stretched)
• 400 x 300 – not ideal (small, requires image to be stretched)
• 450 x 337 – most ideal
• 500 x 375 – most ideal
• 550 x 412 – most ideal
• 600 x 450 – most ideal
• 650 x 487 – most ideal
• 700 x 525 – most ideal
• 750 x 562 – most ideal
• 800 x 600 – more ideal (file size)
• 900 x 675 – less ideal (file size)
• 1000 x 750 – less ideal (file size)

Avoid using BMP files. The PNG, JPG and GIF are the only supported file formats