It means the image you uploaded doesn't match the source you provided. Possible if you uploaded a compressed version from elsewhere and then found the source manually rather than uploading directly from the source by url, or maybe your device compressed it if you saved to disk. In this case there are visual differences, however minor they may be.
In the future you can use the bookmarklet or just paste a twitter/pixiv/etc URL into the source box on the upload page for convenience.
