Generally changing specifically the community blog ad picture, I think, is okay. I highly doubt the community would loudly oppose keeping the Blog ad with a fresher image (unless you changed it to a terrible, terrible image, so you would be wise to just do an overview pass of the image before you change it). The ads now run for a whole year, so getting a new image isn't a bad idea now and then. In this very specific case, I think it's alright.
Changing it for every blog post, however, seems a bit overkill. This mostly comes from the fact we do not support the ability to run "programmatic" ads that update themselves (like our cross-site question ads) - we can only serve static images. Note that I don't think that this classifies as subverting the static image requirement.
In a sense, it is "making this kind of thing work with what we have available", but I think it has the potential to permit a lot more behavior of "repeatedly editing ads every couple of weeks". A fresh image is fine every now and then, but every couple of weeks is pushing it, especially when allowances are already being made. Also hard to justify letting this be a "pseudo-programmatic ad" and not letting others run "pseudo-programmatic ads" for free reign to change the details on the ad at any time - coming up with appropriate restrictions on this seems like it'd make the entire ad system a lot more complex than it truly needs to be.
On the topic of blog article visibility though, we did increase the amount of days they show up on the Community Bulletin. Small steps for now in that direction.