When using Mega menu standard, if you display an image in the menu, the theme will default to using the image from Shopify's Collection image.

If you don't want to use the Collection image and want to display a different image, you can use metafields to specify a custom image.
First, create a collection metafield:
Go to Settings > Metafields and metaobjects > Collections
Click Add definition and configure it as follows:
Name: Menu Image
Namespace and key(required): theme.menu_image
Content type:
One value
Image File

Now, please go to each collection that appears in the menu and add an image for it by uploading the image to the Menu Image metafield you created above.