You're going to want the 'i.' in front of the imgur link and add an extension for the image to show up.
[img]http://i.imgur.com/vACJffO.png[/img]VVV
As for the image, your request is a bit too general. It is important to potential critics to know what you are drawing for in order for them to know what would be best for the situation. A few general things are to not post an image scaled up as the forum has built in scaling, and it makes for a tiny bit more hassle for people to edit. Using a tablet is perfectly fine and preferred by many artists, though using tools that have automatic anti-aliasing and transparency/alpha channels would probably fit under your definition of "cheating" and would make your pixel art look blurry and less readable. Basically if you do a stroke and it has over 10 colors then it is probably a bad tool to use for pixel art. To improve, you probably would want to focus on one piece at a time. For the knight, there are a lot of wasted colors that will be extremely difficult to tell apart such as the two shades of the shield/weapon or the two of the shoulder pad. you can either increase the contrast or remove one of the colors and use of the the other already used shades to define the form. There is a good contrast between the head and body, along with the hands and feet which make it readable. The plume has some banding on it. I would recommend shifting the lightsource toward the top right so you can define the plume while easily avoiding obvious banding while making the piece look more interesting overall, and I would also suggest adding a specular highlight to the helmet and shield to give the impression of metal.
That would be my own personal opinion on it (yes the knight was downscaled a bit off. The upscaling didn't seem to have kept the aspect ratio so the downscaling didn't go well). I also spread out the arms and legs so the space in between will help them seem less glued to other parts of the body. As for the tree, there are over 200 colors and is pretty difficult to judge as pixel art until that is cleaned up. There are a lot of good tree tutorials out there you can find by searching. Basically, it is a good idea to focus on clusters rather than individual detail, then go back to refine as necessary. Good luck.
