Question about using other peoples work/pictures

I think the issue falls under "fair use" which is a sort of grey area. Here is the Wiki discussion of what this means in the US. My personal opinion is that if you are using images in a completely different way than the creator uses them or would ever use them that this is acceptable as long as you are not trying to profit from their work or compete with them in some way. This would result in a situation where the person who made them has no interest in Sims 4 and, if you didn't use the images for the game, the images wouldn't be available for the game.

This use would meet the relevant factors of fair use in my opinion:

1. It would stimulate creativity for the enrichment of the general public without superseding the original artist's item for reasons of your own personal profit.

2. Your use would not substantially harm the original artist in any way. You would not be minimizing their abiltity to exploit their work further in any way they plan to exploit it, and your use of it doesn't harm any potential marketing they're seeking to pursue.

I think no one would have a fit under these circumstances because no one is being harmed. If any of the artists has terms of use saying the work shouldn't be converted for computer gaming without permission then I guess I would say you should ask permission. Generally most artists don't have that kind of policy because they have no interest in converting their art for computer games.