These aren't really secrets so much as photography and drawing/painting fundamentals, all the stuff you'll be able to find online about composition. If you can figure that out you're sweet =3
[The Rule of Thirds]
The most basic of basics that effectively dictates everything else. The picture above is boring. Lovely little location as Shing Jea can do so well, but it's not a good picture of it because the island--the feature of the picture--is dead in the middle, and the horizon line divides it in halves. It's boring and predictable. This is where the thirds come in.
Same location, but this time I've divided it up into thirds so you get what I'm on about. Where the lines cross, those points are the most interesting spots on your picture. You want to put something there, either on the spots or along the line. Here we've got the horizon on the top third line, the island on the top right cross, and even brought in the wrecked ship in the bottom right. It's interesting, just because it's no longer so symmetrical. There are, of course, exceptions to this, but I'll get to them. Stick with the rule of thirds and you're bound to get something nice.
The easiest thing to put on the third line and make an interesting composition. As we've just seen, putting the horizon in the middle is boring. Putting anything in the middle is boring, but we'll start with the horizon. In GuildWars, most often you'll want the horizon near the top, since the sky tends to be empty. However, sometimes, in the Charr Homelands and Tarnished Coast especially, the trees are far more interesting than the grass beneath your feet, so the horizon would look better at the bottom.
If you find that wherever you put the horizon, on the top or bottom third line, you have a large blank area of sky or ground, then pick somewhere else to take your screenie, cos that won't work unless you take it into Photoshop and crop it.
Diagonal lines are great. They can draw attention to your subject, balance the picture and just generally look interesting. In GuildWars you'll find plenty, in the form of shafts of sunlight, mountains, long walls or bridges like the example above, even fences or pathways can work. Ideally have the diagonal on or very close to the corner, but keep in mind the other objects in your picture. A diagonal 'incorrectly' placed will at worst disappear; a badly placed object will stick out like a sore thumb.
Hardly vital, but if you happen to be in a spot with something to frame your image, then by all means take advantage of it! Trees are the most obvious, even dead ones, but statues, rocks or the corner of a building could also make a nice frame.
Only take symmetrical pictures if the scene you're taking a screenie of is designed to be symmetrical. Basically this restricts you to impressive buildings (not the little thatched roof huts in Ascalon, I'm talking palaces, statues of the five gods, sweeping Kurzick staircases. BIG things) and gardens (like Seborhin). Try the picture first with more interesting angles and thirds, but if you absolutely have to take a symmetrical picture, use your interface (I'm thinking the skill bar) to make sure you've got it perfect.
[Stuff to avoid]
This is one of the shots that I call a potential shot. It almost works, the composition is technically right, it's an interesting subject, interesting colours, but something about it just doesn't fit. There are a couple of things in the world of GuildWars that just won't 'photograph' well.
 Shapeless rocks, like the mass on the left there. It's just an ugly black mass, and it doesn't help that the otherwise sexy diagonal line of the top of the cliff points straight to it.
 Big cliffs. For some reason, the rendering on near-vertical walls never quite looks complete. You can especially see it in the middle of this shot, just to the left of the palm. See how it kind of looks like a smaller texture that's actually been stretched to fit on the cliff? Yeah, not pretty, which is a pity because there are so many chasms and canyons that would otherwise make for stunning shots (I'm thinking the one in the D'Alessio Seaboard outpost in particular).
 People and monsters. The monsters are easy enough to deal with, just kill everything in your shot. NPCs you'll just have to cunningly hide behind a bush or rock, or wait for them to wander offscreen. Sometimes they'll just not move, but so long as they're at a distance you should be fine. For other players in towns and outposts, especially in big places like LA or Ascalon City, try out the non-English speaking European districts until you find one with nobody in your screenie. There's bound to be one.
 Bits of land in the corner as in the screenshot below. Pretty easy one, just something to keep in mind.
And~ that's about all I've got. Basically, stick to the rule of thirds and you'll be right. If you want anything more, try Googling photography composition and you'll get plenty. Hope this has been helpful!
General Nimay [Snapshot Henchman]
I Got Girl Parts [ZOMG]