Hi Mike, (and welcome)
First of all, don't talk about images as they are 2D raster images and we are talking about 3D vector based modelshere, right?
Now as for imported models; they are most probably scaled right (I mean 1:1) as you shlould aslo make your models. So first of all, make sure that you are modeling in the correct scale (1:1). To make sure, use the tape measure tool to see how big YOUR model is (I know it's all virtual but still we need a refernce to "real world" in virtual reality as well).
If something is out of scale you have two choices:
use the tape measure tool to scale a group/component or a whole model use the scale tool to scale any geometry (primitive/group/component).If the conveyor belt is the "culprit" (being out of scale) of course you need to scale that, not your model.