Sorry to hear you are having issues. But, I have no problem importing my own image into skinstudios, save and applying them. Not really sure what your steps in doing it. But here what I usually do.
1) Save an image that I want to use somewhere. Usually PNG or BMP
2) in Skinstudio -> click that ">" dropdown button next to the "Build high dpi image" -> Click import. ->
3) click "..." (browse button) -> go to the location you save that image -> select it -> Ok button. Sometimes it does take sometime for your image to appear in that preview area. Sometimes need to click another section and comeback to see if it changed. The naming of the graphics file does help. Usually I just add numbers at the end of that name for revision purpose.
4) Do any adjustment on it if require i.e margin setting, text colors etc etc.
5) Save skin.
6) Apply skin and see if its good. If not repeat.
7) For back up purpose, File -> Make WBA package. And make a duplicate copy of it. Just to make sure you have something to turn back to if you need it.
Stardock Community Assistant.