Simply put, images made in GIMP are invisible in MCreator itself (except in the texture editor for some reason), but they are normal in Minecraft. Images demonstrating this of a block I made (heavily cropped in the MCreator tabs so I don't reveal everything I am working on before I release it) can be found in the attachments.
1. GIMP export configuration (may help with figuring out what is causing the bug)
2. How the texture appears in most of MCreator's menus
3. How the texture appears in MCreator's texture editor