word choice - Compare 2 factors among others

I am trying to polish the following sentences:

The analysis time of an image depends on many factors. The size of an image is not as primary as its nature, because...

The context is:

1. There are many factors which determine the analysis time of an image.



2. Among them, we can think of "the size of an image" and "the nature of an image" (but there are still other factors).


3. For these 2 factors, our remark is that "the size of an image" is not as important as "the nature of an image".

So my questions are:

1. I'm not sure if "nature" is a good term to use for "image"...


2. is "primary" a good word here? or maybe we can use "decisive"?

Hope someone could help me write this sentence well, because it is a conclusion and quite important.