Just wanted to make some comments about posting:
I don't think it's bad to reply to old posts. Secondly, if someone's posting new material to one of their own posts, and it ages to the point where it falls off the page, I think that double posting is the more responsible action to take, versus creating a new thread that only adds to the 'noise' on a forum.
When we use the term bumping, it takes on a shade of negativity. The general feeling seems to be that if a thread falls off a page, it's obviously done with, and that new posts deserve the attention. But when a forum grows large, there's so much topic movement that it's just a natural part of the chaos that we have to get used to. Granted, a larger forum means more inexperienced artists posting works which, quantatively, means that the forum has less 'good' art, and such people might be better off doing more studying before they post (a more strict/helpful signup quiz could resolve this?).
It sounds like some people are of this mindset: When old posts are getting 'bumped', my posts lose visibility and that takes away from my chances to get a reply. That's more than likely true, but it's also a theoretical perspective of an individual wanting to get their own needs met.
You might want to only give quick praise in a reply. That's great, nobody should dissuade you from posting at all, unless that becomes a habit, because people may tend to ignore that if they see someone who's always praising but never criticizing! It is best to at least try to focus on the ideal of constructive criticism. Good critique is just as challenging as creating good pixel art and it's something to practice at.
When I give more effort into a critique, I am benefitting the forum on the whole:
1. The chances that the artist learns something and improves are increased.
2. The resulting works from that critique are likely to be better, thus we now have better work on the forum by a better artist, who through solid critique has learned how to judge art as well, rather than how to suck up to someone.
3. The quantative value of good art on the forum increases, creating more interest, as well as friendly competition to self-improve.
4. But most importantly, the number of available 'good' artists available to respond to threads is increased, which cuts down on the delay before demand is supplied. We surely can't all be here all the same time to answer someone in need, but at least more people who have benefitted by your wisdom ought to be there.
So, to conclude: Critique isn't ever easy, but whether you feel like giving it or not, it's worth it.