In image.ini, there is a misleading default configuration between ConversionRules and AllowedOutputFormat.
ConversionRules says that image/gif files should be converted to image/png. But this will only happen if AllowedOutputFormat doesn't contain image/gif, while they are allowed by this rule.
The consequence is that even though it seems, based on the file comments, that GIF files will be converted to PNG, they are not.
The workaround is to remove image/gif from AllowedOutputFormat.
This must be clarified.