I belive the emission angle is the true angle the images are taken, in this case from nearly directly above. Some images are taken from an angle, even 20 degrees off nadir and adjusted to simulate an overhead view. With some features though this transformation is inaccurate. For example if these "geysers" were taken from a large enough angle and transformed then it might well appear like this image.
However a geyser should point straight up at the camera and not be seen as in the image, hence it is likely to be an optical illusion. Even if the wind blew it sideways enough it still could not give this shape, or appear as more than a streak from above.
Also if you look at the amount of dark material supposedly in the geyser then it is often 10 -50% of the total amount seen on the ground. I would submit that this is impossible since in perhaps a minute more material spurting out would make much larger pools than the geyser size. I would expect the material around the geyser to be perhaps a hundred times larger than it. The only other explanation would seem to be the dark material disappears on contact with the ground (though dark spots are not known to disappear) or it is covered up quickly. Since the theory is that CO2 is sublimating I doubt it could simultaneously be covering up dark soil with light CO2 snow. In fact dark spots are not known to disappear like this.
To me these two aspects make it impossible for them to be geysers. If they are geysers then this is nothing to do with the visual evidence. I don't have pictures on hand (I will check) but it is extremely common on the North Pole for areas of dark soil to be covered by light CO2 snow or ice. Then small patches get uncovered and in turn heat up and sublimate the CO2 in expanding dark spots. This is known because later the dark soil (often in the form of a Barchan dune) is completely exposed. There are also plenty of examples of dark soil areas on the South Pole after the CO2 has gone, so an analogous process to the North Pole is likely. I can provide images of all of these.
Sometimes dark spots like these have a "grey" area around them but this is likely to be partially sublimates and thus thinner CO2 ice around them.
I am skeptical of the idea of transparent ice in these models as well, since by definition they invoke an invisible material to explain something.
It is also known there is a strong wind (by Martian standards) blowing away from the pole as the CO2 sublimates, indeed this increases the Martian air pressure substantially. It is further known this dark soil is often blown around by Martian winds, there are many examples of this without any CO2 to complicate the perception of the process. It is also known the wind changes direction as the CO2 sublimates and freezes. I can give many examples of streaks pointing in different directions in the same image. We see the same thing on Earth in areas heating in different rates.
So I submit these are all facts or at least simple explanations. So Occam's razor then explains these images by spots of dark soil exposed through white CO2 ice, then blown around at different times by changing wind directions, forming 2 streaks from each spot.
But Nick's main interest I believe is the model of CO2 forming certain features, as part of an overall theory of Martian geology instead of water. So degassing CO2 might form avalanches of CO2 ice and form ravines over time, as in his image. I doubt though the geyser theory is needed to support this idea. CO2 ice might be as prone to avalanches as water ice in mountains on Earth.
There is also a problem with the spiders being supposedly formed by degassing as that implies joints along which the gas is explelled. If the dark spots are also geysers of CO2 the gas should find its way into the spider fissures. However these spots seem completely randomly placed near spiders and there seems to be no tendency to connect to them. There could be something to the idea of dark spots and spiders being two different forms of degassing, occuring for example in different kinds of soil. Spiders might form in fissures in harder ground and spots in softer soil which doesn't crack. In fact there are many areas where spots and spiders form separate areas and appear to be a transition from one to the other.
I haven't been looking at all this for a while, but that's the best of my recollection of the processes involved.