My problem is that I cannot view an image loaded from an external site into an img tag in my lightning component. The URL is with 'http' protocol not 'https', but when I see the Chrome Console, it tells that failed to load resource and I noticed that Salesforce try to go to the external site with the 'https' protocol and so it not find the resource. Why all this happens?
As @ryangreene has commented in a different answer the solution may be found in https://help.salesforce.com/articleView?id=000334127&type=1&mode=1
Chrome has a security feature to block insecure content (HTTP) which is being loaded via a secure page (HTTPS). In order to avoid this error, the content being loaded must be in secure form (HTTPS).
How to bypass the Chrome limitation could be found here: https://stackoverflow.com/questions/18321032/how-to-get-chrome-to-allow-mixed-content