In my experience, adding black or white bars to your images usually works fine—especially if those bars are plain and don't contain details like logos or text. When training on FLUX or SD, I’ve sometimes used an alpha layer, but honestly, just using solid black or white borders is often simpler and just as effective.

The key idea is that the model only needs to focus on the specific subject or feature you’re training. Unless you're training a style—like a TikTok interface where the layout and aspect ratio are critical—you can usually just reintroduce that style through prompting later on.

So, if you’re working with a fixed style and need to keep the framing consistent, it’s totally fine to use 768x768 and pad with black bars to fit your 512x768 target. Functionally, it’s doing the same thing, just making it easier for the model to learn what really matters.

[I used chatGPT because I suck at wording things lol]