If the image files you need to show do not happen to also exist in a server you own or operate (typically where your ecommerce solution is hosted), you can host (upload) them in an online service offering specifically this functionality.
In all cases of such services the first step is to upload all your product images to the space online the service provides. Make sure the files are appropriately named so that you know the product (incl. version, variation, etc) they refer to.
Then you need to include the correct image URL in the respective field of each product. This can be done in bulk by Importing the Documents. You also need to make sure the links added in Megaventory is to .JPG, .PNG, .GIF or .BMP files.
Currently one of the services supporting this feature is https://postimages.org/
Note: It is not possible anymore to upload your product photos to the popular file-sharing applications Google Drive and Dropbox and show them in Megaventory.