You have to take the photo’s yourself. 
Hi!
I don’t know if I’ll have time to participate this month but in the case of I have some question.
So no fancy Photoshop tricks are allowed I get it.
But can we do a quick adjustment with the level to erase some background artifacts in our pictures or we have to deal with?
For color , “particle color” parameter is driven by shader so can we change the color this way?
Or I have to take a photo of a color or a painted gradient and use it in RGB?
That can influence my potential ideas . 
No levels, or erasing, embrace your mistakes! (or retake the picture)
Yup, you can change the color using the particle color parameter.
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Can we use our tablet to paint a fancy alpha mask for it? Or does it have to be a hard crop around the image?
are photos from a scanner acceptable?
@JangaFX Fancy alpha masks are ok, you just can’t use that mask as the colour input.
@DaDarkDraon Yup, using a scanner is totaly fine.
Ah, also,
You are allowed to use multiple photographs, and thus multiple materials to make your vfx!
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Are flipbooks / sprite sheets allowed? 
Also, I’m assuming that postprocessing like bloom, color correction, chromatic aberration, etc. is allowed right?
yup, flipbooks etc, are allowed
the material restriction is only on particles.
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Can I scroll the texture over a ribbon mesh? or do I have to use trails?
UV scrolling is shader manipulation, so nope :D, not this time.
Hold up, so is that a no to trails as well? D:
You can generate trails etc, but the material cannot scroll over the trail.
you’ve outdone yourself @Niels 
I’m psyched!
Quick little question:
Can I use emission?
Can I use Fresnel in emission?
Can I erode the environment using the texture alpha?
Thanks )
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You can use emission to get your texture to the particle.
You can’t use Fresnel (unless you somehow figure out how to do it without a material)
Same as above, if you can do it somehow without using material tricks, then yes, otherwise no.
Good luck