PHP Header

header('Content-Type: ' . image_type_to_mime_type($image_type));

where $image_type is any one of the IMAGETYPE_XXX constants as listed in http://php.net/image_type_to_mime_type.

IMAGETYPE_GIF	image/gif
IMAGETYPE_JPEG	image/jpeg
IMAGETYPE_PNG	image/png
IMAGETYPE_SWF	application/x-shockwave-flash
IMAGETYPE_PSD	image/psd
IMAGETYPE_BMP	image/bmp
IMAGETYPE_TIFF_II (intel byte order)	image/tiff
IMAGETYPE_TIFF_MM (motorola byte order)	image/tiff
IMAGETYPE_JPC	application/octet-stream
IMAGETYPE_JP2	image/jp2
IMAGETYPE_JPX	application/octet-stream
IMAGETYPE_JB2	application/octet-stream
IMAGETYPE_SWC	application/x-shockwave-flash
IMAGETYPE_IFF	image/iff
IMAGETYPE_WBMP	image/vnd.wap.wbmp
IMAGETYPE_XBM	image/xbm
IMAGETYPE_ICO	image/vnd.microsoft.icon