For the Gallery (Card) image and large image in the first List item:
- The recommended size is 909×476 px.
- They both use exactly the same aspect ratio, which is 1.91 (width) to 1 (height).
- The images must be at least 500×262 px.
For images added via the Image block:
- The recommended size is 900×900 px but they must be at least 450×450 px.
- Dimensions can be lower than 450px, just don’t make one side longer than double the size of the other side.
- The closer your image is to a perfect square, the better.
- A wide image is better than tall one.
- These images have dynamic aspect ratio and they appear differently on different devices.
This information was extracted from an article published by ManyChat.