What is the impact of using one image for several post on my site? Does it have a negative affect on SEO? For several posts, I use one picture. because all of them are Q & A. Am I wrong? is it bad for SEO?
Using the same image in several places isn't necessarily bad. I think it can actually be good UX if you use it for a recurring type of post. That way, your users will easily recognize this type of post while scrolling down your blog.
In terms of SEO I would consider if you can make the image more specific to each post and/or keyword (if that's possible with your post titles). For example, you might consider changing the ALT tag on a per-post basis or even download the image, change its title and re-upload it for each post so you can use different keywords.