Uploading a skin and later combining with tattoo - how?

I uploaded the skin textures and applied them to the avatar by the means of the "tattoo" maps.
However, now I want to place some tattoos on the av, which will surely replace the present maps.

Does that mean I need to combine the tattoo with the skin in image editing software, and later re-upload all of them?

Thanks in advance for answers.

As far as I know- yes.

As an interim measure you can also apply your tattoos to clothes layers (usually undershirt and underpants) unless you want/need tats on the hand (gloves - they'll also do arms of course to about mid-brachum) or the face (can only be done on the tattoo layer).

Whilst many see this as sub-optimal (you can take your tattoos off after all this way, and it will clash with some clothing choices) some prefer it. I have a limited range of tattoos and don't wear them often, but I have some outfits that go wonderfully with tattoos that come with them or other sets and this way I get to choose without having to chop and change skins.