Apparently, GIMP doesn’t like Smile Game Builder much. Images created, rendered, saved, exported and imported in SGB in exactly the same way as normal. Yet 50% of images become corrupted. Images saved with other graphics programs don’t "corrupt" this way when displayed in SGB.
Here’s a normal picture rendered through GIMP:
And here’s another image displayed in the editor:
It’s quite an anomaly, isn’t it?
Why does this occur?
Well, it’s due to interlacing. As part of its PNG compression process, GIMP uses the Adam7 interlacing algorithm, which breaks images into 7 subimages in an 8×8 replicated pattern. This method is incompatible with SGB, so the results become corrupted.
The solution, therefore, is to uncheck this when exporting images for SGB through GIMP.
Here’s the same image as #2 above, but without the Adam7 interlacing:
As you can see, it’s now uncorrupted.
So this is something to bear in mind if you use GIMP (or other graphics software). If images become corrupted, try re-saving them but with interlacing off. Or re-render them with a different type of interlace.