Failure to adhere exactly according to policy can and will result in a block. Pay attention.
Wikipedia is as the slogan says, "The Free Encyclopedia". Unfortunately, this causes some problems when we use other materials that aren't so free, and other problems when we'd like to do something but really can't. Most of Wikipedia's text and many of its images are co-licensed under the Creative Commons Attribution-Sharealike 3.0 Unported License (CC-BY-SA)
and the GNU Free Documentation License (GFDL)
These are copyleft licenses that allows for the free distribution of content under certain conditions. The main terms of this license are as follows:
There are other terms to the license, but those are the most important for what is done on Wikipedia. Wikipedia displays a copy of the license, which is fully protected under the authority of the Wikimedia office. Whenever we make an edit, that edit is logged in the page's edit history, as well as your contributions. When a page is deleted, contributions to that page are hidden, but are still visible to administrators or "sysops". Certain page revisions may also be hidden from public view in the event of extreme circumstances, but are still visible to those with the authority to remove them for CC-BY-SA and GFDL compliance.
Unfortunately, these licenses does have some limit on what we can do. When merging pages, we cannot delete the page that is now empty, even if it serves little useful purpose even as a redirect. The contributions to that page, which provided the information that was merged out, must be kept logged so that people know where it came from and what changes were made when.
Problems such as those, however, are rare, as the Mediawiki software is designed to be GFDL compatible. (As a side note, the software itself is available under a similar license, the GPL.) The most common issue, and the one that most frequently results in blocks, is copyright. Any registered user can upload an image or media file. You've done this yourself, with your logo on your userpage. If they created the image, as you did yours, they can license it under a free license such as the GFDL or a Creative Commons license, or release it into the public domain (Although if you use any of those options, it's recommended to upload the image to the Wikimedia Commons instead so any language Wiki can use it.)
Problems arise when people upload images that are not their own. Most images are under some form of copyright, even if it's not explicitly stated anywhere. This is usually the case with anything found on the internet. When these images are uploaded, Wikipedia must adhere to a very strict policy known as "fair use". What this basically is doing is giving us a reason to use an otherwise non-free image, on the basis that it is for educational purposes, using it has no measurable effect on the copyright holder's rights, and that we have no other alternative. The establishment of this reason is called the fair use rationale, part of a set of criteria that MUST accompany any fair use/copyright tag on Wikipedia. These criteria are:
Only when an image meets all of these criteria may it be used. Fair use images must be used in at least one article (not "orphaned"), and articles using fair use images must use as few of them as possible. Any image that does not meet these criteria to the letter will be deleted. Any user that repeatedly uploads images not meeting these criteria to the letter will be blocked.
As a further note, I mentioned that fair use images must not be able to be replaced by a free alternative. What this basically means is, there is no way you, me, or anyone else could go out and take a picture of this same thing and release it under a free license. For example:
For a full description of the policies and guidelines concerning fair use, you should read the page at WP:FU.
After reading all of this, write a really brief paragraph on my talk page about what you learned from this lesson. There are no right or wrong answers, I just want to learn what your understanding is.