There are technical issues, copyright issues, ethical issues and creative issues to consider.

Technical issues
1. The image will need to be scaled up in size. With the right software you can get good surprisingly good results. I printed a 1200 px wide image on A3 with more than acceptable results.
2. It is very dependent on the type of image. An image with many sharp edges and fine detail will not up-size well.
3. A canvas or semi-matt print surface is more tolerant than a fine glossy surface.
4. Placement has an influence since that will often control the viewing distance.

Copyright issues
As you noted, we should always respect copyright and ask for permission. One can make a strong argument that the greater good of society is best achieved by freely sharing creative work, which is after all what happened until 200 years ago. But the law, as it stands, does not support that position and becoming a scofflaw is not in the interests of society. I personally recommend that people publish their work under a Creative Commons licence unless photography is an important or main part of their income, in which case it is to be expected that they reserve all rights under copyright law.

Ethical issues
If you display work which is not your own you should always show the name of the author. If you don't there is an implication that it is your own work, which is decidedly not ethical. The author normally expects some kind of recognition which usually takes the form of attribution.

Creative issues
This comes down to asking the question, why are you, as a photographer, displaying someone else's work rather than creating and displaying your own work? There are good reasons for doing it. Other people's work can provide a powerful creative stimulus and source of ideas. All artists learn from their fellow artists work. We can use it as a reminder and an incentive to do better work. When we admire other peoples good work we enter a zone of openness and humility that allows the creative process to flower.