@ScottMorrison: I thought given that there are a couple of nice answers, I probably should choose one to "accept". Since I had no particular feeling about which is the "best" of the given answers, I just took the one with the highest number of votes. Though, thinking about the issue, I agree with you and Benjamin that accepting an answer probably makes little sense here, and hence took it back.

I think this throws up a more general question, namely whether in times of the internet it is really appropriate to make a contest with publicly available questions which are the same for every participant. -- As soon as the questions are published, with whatever motivation anybody in the world could post answers on whatever website, and participants of the contest could easily find them. Also, the legal possibilities for the organizers of the contest to request deletion would probably be quite limited.

Thank you very much to everybody for all the interesting answers and comments!

MO allows only one answer per question to be "accepted". Thus you need to decide.

+1. -- Though I wonder whether there is a specific reason for bringing this up as a question?

Why not make the entries in the "active" tab itself more verbous? -- Presently they say only "modified xyz mins ago". Wouldn't it be reasonable to make this read "new answer added xyz mins ago" if a new answer was added, or "edited (+X/-Y characters) xyz mins ago" if the post was edited?

As you mention images: is there a particular reason why images are not just treated like text, but need separate hosting?

I hadn't read the question as asking for a way to include the text of a paper into a question. Rather I thought the OP would like to attach a file to a question. But maybe I am wrong.

Is there a particular reason against uploading the PDF file to some other website (e.g. your homepage) and providing a link in your question on MO?

This is certainly reasonable if only a snippet of the article is needed. -- But posting an entire article in this way is maybe less so ... .

@ManfredWeis: regarding your "LP/ILP" integer factorization: just implement it, and if it beats FactInt (cf. gap-system.org/Packages/factint.html) for sufficiently large and difficult integers, send me the code and I'd be more than happy to check.

@quid: By the way, I remember having heard that some people translate the "GM" in math.GM on arXiv as [G]arbage [M]achine, reflecting that at times arXiv moves low-quality papers from the more specific sections there. I wonder whether it is generally perceived in this way, and whether people might read the "GM" here on MO in a similar way.

Presently, gm.general-mathematics has no tag wiki, and at least to me it is not clear how to understand "general mathematics" -- maybe in the arXiv sense?