What triggers a texture to begin loading?

Does it have to be in line of sight?

Does the object it's on have to be non-alpha?

I have several scripts that switch texture in response to payment, and some customers are frustrated that the texture switches to the grey "loading" one which then remains until time runs out. I want to know if there's a way I can force the textures to preload, possibly by sticking them on a tiny, blocked or 100% alpha prim?

I stick 'em on hidden faces. Making the texture alpha does not seem to affect this. Tiny objects need to be close to rez at all, unless linked to a larget object.

The texture doesn't actually have to be visible to load - It's governed more by how close it is to the agent (camera) than anything else.

There's two methods to achieve what you're after:

Sit two prims one-behind-the-other and allow both textures to load at the same time, then set the front one to full transparency when you want them to change. (Useful for animated textures - IM me in-world if you'd like to see this in action).

Or

Edit the textures in (insert favorite graphics package here) and paste them both onto a larger texture, so that they're split exactly 50/50 across the image. When you apply them to the prim adjust the repeats and offset to only display the half you want. Both images will load at the same time and at the point you need the change, just use your script to change the offset to display the other half of the texture!

Let me know in-world if you need help with either of these methods.

Good Luck!