I have had some issues with this as well. @AOU said it has to do with pic8 being stringent about the file format. One in particular a few days ago I had to resort to screen capping the pic and then posting the screen cap. The issue in my case had to do with oddball color settings from a source pic.
Most image viewers are too tolerant with corrupted image files and since pic8 strips off the exif/metadata to preserve privacy before uploading to different hosts, a corrupted image file will throw that error.
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