A flaw was found in the GIMP PSD plug-in thumbnail loader. When a PSD embeds a grayscale JPEG thumbnail, the BGR-to-RGB swap in load_resource_1033() uses a hard-coded 3-byte stride without checking the JPEG component count, reading and writing past the heap buffer. The overflow length is controlled by the file. This can be triggered during thumbnail generation in the file chooser, without fully opening the image. This is distinct from CVE-2026-92248 (thumbnail allocation integer overflow).