For me it depends on the icon and the crop: if it's a negative space icon, the main texture goes behind the focus object (aka the person). If it's not a negative space icon, the texture goes on top of everything (or at least on top of strategic parts of the icon).

I make a lot of negative space icons because I feel like the background is sometimes too busy and distracts from the focus of the icon. But just leaving things blank is a bit boring, so I go looking for a texture to work with the icon.

If it's not a negative space icon, it's very often a close or mid crop icon (I don't like full body icons, so I rarely make them unless prompted), meaning there is very little room around the edges for a texture unless it's on top of everything. So in these cases I pick textures that make the icon a little less bland and boring (like in the first example icon you gave) and that work well with the screen or overlay or soft light blend modes. I use light textures a lot, too. Sometimes though I want to obscure parts or make a certain statement like in the second example icon - the texture obscures Donna's eyes and it reminded me of the season one finale where Rose looked into the heart of the TARDIS; I thought it was fitting for the Doctor Donna. *g*

Occasionally I have a case like this one:

Technically the texture could have gone behind Flynn, but I remember that tried it with several different blend modes and with the texture tilted this way and that way, but this looked best.

I also generally try a few different textures and look at them side by side to see which texture works best with the icon.

(You might also want to check out this comment because tinyumbrella asked a similar question.)

I hope that answered your question!