How can we find in which image types are supported by the current PHP installation ?

Using imagetypes() function to find out what types of images are supported in your PHP engine.
imagetypes() -Returns the image types supported by the current PHP installation.
This function returns a bit-field corresponding to the image formats supported by the version of GD linked into PHP. The following bits are returned, IMG_GIF | IMG_JPG | IMG_PNG | IMG_WBMP | IMG_XPM.