Use the IsImage function to test whether an image variable represents a valid ColdFusion image. This function takes a variable name as its only parameter and returns a Boolean value.
Note: You cannot use the IsImage function to verify whether files are valid images. Instead, use the IsImageFile function.
Also, ColdFusion provides two tags for determining which image file formats are supported on the server where the ColdFusion application is deployed: GetReadableImageFormats and GetWriteableImageFormats. For more information, see the CFML Reference.