The "uhauhauha" jokes need some context, else you won't find them funny anyway.
As a Brazilian I find them funny because it is so ridiculous and recently, they are being featured internationally. If you imagine someone laughing like that, you'd certainly think they are absolutely mad, crazy. The images make a perfect representation of somebody laughing like this.
For context: in English, people usually say something like "lol" when they're "laughing". Most Brazilians on the internet think it's somewhat stupid and doesn't represent intensity, at all, specially being so overused. "haha" or "hahaha" don't quite get it either, and there are stupid people who write "rarara" or don't quite get the "haha" (in Portuguese, H doesn't have any sound at all in this syntax, only if preceded by another consonant such as S as in "Shy". "Hello" would be pronounced "ello").
So, most people usually laugh on chat with something like "huauhauha" or "uheauheuh" (it's random and every person has their own way of typing it).
It may involve randomly hitting, though in a somewhat equally distributed way, keys h, a, u and e (may also include i and o) during the period of the laughter. There are people (who most Brazilians also consider retarded) who laugh like "aopkapokapo" or "rairairiaria".
One thing that has become most common is just "kkkk".
You may be wondering, "how do you represent intensity?". That's simple: more k's, more intense laughter, as in "I'm still typing the laughter as I laugh, so funny and exciting it was!". Put them in upper case and you're almost crying.
Variations, from most simple to most complex:
kkk
KKKKKK
huahauhau
hueuhehue
huaehuaehu
aeiuhaeiuhu
aeiohaeuioh
aUHEAuheaUH
oaIUEHAioehaIOEH
UHAUHAUHAHA
AIEHAIOUHEAIOUEH
There's even some sense into them:
k - not funny, sorry
kk - shy smile
kkk - yeah, funny
kkkk - yeah, that was funny!
kkkkkkkkk - "HAHAHAHA"
KKKKKKKK - ZOMG THAT WAS HILARIOUS
But the length of the string depends on the person, and you figure out how much the other person is laughing by context. For some people, "kkkk" is laughing a LOT. For others, "kkkkkkkk" is normal, "heh, funny!' laughter. The most overused it gets, the less the person is laughing. Quite simple, actually.
Not all people are retarded, though. In Brazil, when you want to look respectable or to impose respect, you laugh the minimum like "haha", "kkk" or don't laugh at all. It should be implicit from context whether it was funny. And you must be consistent. If it really is incredibly funny, put ONE exclamation mark.
To sum it up:
Retarded Brazilian
POAKPAOKAPKOOPKAOK MASSA VÉI !! == ZOMG ROFLMAO VERY FUNNY
("OUAEHOAEUHEAOUH MUITO ENGRAÇADO!!" also fits, which is the very purpose of these jokes) Normal Brazilian
uhahuauha isso foi engraçado == lol, that was funny Respected Brazilian (notice first-letter of sentence in upper case)
Haha, boa. == Haha, good one.
Some people need to understand that "funny" can mean anything, some people like something else than others. If you do not like someones "funny" post then stfu no need to complain about it, we might not like your shit either.