The easiest and fastest solution is to use the URL of the image in question, if you have it. Make sure the URL starts with http:// and ends with an image file extension (.png, .jpg, .gif etc.).
If you have made an export, you will see a code (for example !ID:1996818) listed, instead of the image URL. Each image has its own unique code within Lightspeed eCom. These codes are listed in the same order as the images associated with products. If you know where the image is located and with which product, you can retrieve the code from the sheet. Use it to add it to the sheet for new products. Note: Codes always have to start with !ID:.