Welcome to Shopify Community. Thank you for sharing your question with our network and for adding that helpful screenshot.
In addition to the context you have shared, it appears the specific error message is concerned with Image Src being blank as opposed to "image ALT" as you mentioned. Have you clarified that the values entered under the Image Src column are readable in your product CSV? This should look like a URL for the product image already hosted online.
To better support you, can you please upload an example of the product CSV file you are currently using? Please feel free to remove real product data, and just leave one row filled as an example or test product.
I look forward to supporting you,
Olivia | Social Care @ Shopify - Was my reply helpful? Click Like to let me know! - Was your question answered? Mark it as an Accepted Solution - To learn more visit the Shopify Help Center or the Shopify Blog