Without their permission, yes. One of the exclusive rights creators get from the law is the right to copy. By downloading it, you've copied it from a server to your local machine.
The picture itself is protected by copyright; downloading it without permission would be copyright infringement.
Yes; it would be considered creating a derivative work, which is one of the exclusive rights of the copyright holder.
Crediting a source will not exempt you from being charged with copyright infringement.
If you are caught, According to the U.S. Copyright Office, you could be fined between $750 and $150,000 for each instance of copyright infringement you're convicted of. The Motion Picture Association of America (MPAA) says that you could be slapped with $250,000 in fines per film or television episode you steal, and up to five years in jail. Often, those who are accused of copyright infringement via illegal file sharing are offered the opportunity to settle their cases before official lawsuits are filed.
In the US, the copyright holder can sue for up to $30,000 per infringement, or $150,000 if willfull infringement is proven. That being said, most copyright disagreements are settled long before they reach court, for an amount much closer to real damages.
Not if you properly attribute it. It is more likely to be copyright infringement, however, which is often considered to be worse--it certainly has larger fines.
Yes, it is generally illegal to post someone's picture without their permission, as it can violate their right to privacy and potentially lead to legal consequences such as invasion of privacy or copyright infringement.
No it's called counterfeiting.
Technically yes it is. Because your edited photo required the original in order for you to make your derivation you would be guilty of infringement. Now as a practical matter the scenario you lay out makes it impossible for you to get caught however we're talking about copyright law here, not practicalities :-)
Yes. You've infringed Marvel's exclusive right to copy the image first by downloading it, and again by printing it.
Not if you own the VHS copy and are copying it for personal use. By law you are allowed to make a "back up" copy. As long as you are not making multiple copies and distributing them there is no infringement.
Dozens, if not hundreds. Every time you create something new, it is automatically protected by copyright. Doodle a picture of a cat, you have copyright. Take a picture with your phone, you have copyright. Record yourself making up a song, you have copyright.