It is generally possible to use AI-generated images from Midjourney and other sources commercially without facing copyright issues, as long as certain conditions are met. According to Midjourney's license, you can use your Midjourney image assets for any purpose, including commercial purposes, as long as you comply with certain conditions. One of these conditions is that you must not claim ownership or authorship of the images or imply any endorsement by Midjourney. Additionally, the US Copyright Office has stated that you cannot copyright AI-generated images, as they are not considered to be original works of authorship.
However, it is worth noting that the laws regarding AI-generated images are still evolving and may vary depending on the region. As a result, it is important to be aware of any applicable laws and regulations in your area, and to consult with a legal professional if you have any concerns about using AI-generated images commercially. It is also important to be aware of any specific terms or conditions that may apply to the use of AI-generated images from a particular source, such as Midjourney.