If you're posting something to MoFo using a host site, like IMGUR for example... make the image no wider than 700 pixels and it won't be adjusted by MoFo's white text boxes to fit the site... and it will look crisp and clean. A good height for an image on MoFo is 500 pixels and that way it will fit the height of the page as well.
Height of an image isn't too important, but 700px wide is as wide as I'd recommend.


Actual file size isn't a problem on MoFo if you're using a hosting site... but the host site itself might not like huge files.
IMGUR hates BMP images... but, it works superbly well with PNG.
PNG is far superior to JPG images as well. PNG is loss-less format, meaning unlike JPG, they don't pick up digital noise when shared between sites and devices.

Example.
Ignore the topic of this image... the topic is redundant.
Using IMGUR:

This first one, is 700px wide, 500px tall. It fits in the white text area without MoFo needing to adjust it.
https://i.imgur.com/zS48ug7.png


This second image, like the first one, is 500px tall, but is twice the width at 1400px wide.
As you can see, MoFo has had to squeeze the image and shrink it down so it fits inside the white text area, meaning it has lost some of its coherency and look a little blurry.

https://i.imgur.com/JwuW1f4.png


Both images are PNG, which is why the first one looks kinda jaggedy around the edges of the writing and there's no colour bleeding at the edges.