I found some tutorials online on using Skininator and followed them to the best of my ability without Photoshop. Whenever I import my skin into game, it shows up fully black yet the face is fine. I've read that not only does it have to be exported as DDS but needs to be "DXT3"? I'm not sure where to make that change as I use a free Photoshop alternative dubbed Photopea. I'm also familiar with Gimp and would love tips on how to do this if that would be a better program to do this in. Anyway, I'm making a Dalmation skin for my sims and the spots wouldn't show before until I played around with the alpha channels and loaded them into Skininator as a body overlay and they show up now...but they're blue and the skin's still black. I need help ^^'

I fixed it and am sharing what I did for those who're encountering the same issue with no help.
1) I exported my file via GIMP. If you use GIMP and don't have the necessary DDS options, download the plugin. Here is the youtube tutorial that helped me do that.
2) Click File > Export as.. > Select File Type > Choose DDS
3) You'll get a pop up "Export as DDS," in it you'll choose BC2/DXT3 for the Compression, under Save please choose All visible layers and Generate mipmaps under Mipmaps
4) Click export and import it accordingly by age and gender into Skininator. I followed a helpful tutorial for that as well, here.

Note: The mistake I made is worrying about the body overlay section since I had designs on my skin. I left the 'Body Overlay' slot in Skininator empty and my skin showed up normally! Again, this worked for me.

You're welcome!