A number of them will be being used by search engines (such as google) to update their indexes of PPRuNe.
I doubt google opens dummy accounts to index posts.
What would be the purpose of having a bot that posts parts of previous posts and includes a 1x1 pixel sized image that is not visible? If it was an attempt at spam would there not be links or something visible?
That's a popular tracking trick. Each time someone reads the page his browser will have to call for the 1x1 image from the host site. Once the request goes, the webserver knows what IP you are browsing from (unless you've gone out of the way to prevent this) . So spammers / trackers can figure how many readers and from what locations.
This is the way with email spam anyways. Not sure if it works the same way with forum spam. With email spam, oftentimes each email has a slightly modified link all redirected to the same inviisible 1x1 image. That way they can know exactly which of their thousand spam Viagra emails has made it to human eyes.
