flame particles are not flames. Look at my inventory to see the difference. I have a flame and a flame particle. They are 2 different items.

so you need both to make the flame visible?

no, you only need the flame item to make it visible
the flame particle is just used to make the design different on the flame
but you dont need the flame particle to make the flame itself visible

If you dont have the flame particle item, then you will ahev the basic flame, that means the basic flame particle, which can be found in your uke's custom folder.
A flame particle is just an addition to your flame

So what images can I upload to it?

Here you are.
This is from uke's custom folder, as I said.