While downloading images from internet, it's easiest to let php decide what is the file type. So, forget using imagecreatefromjpg, imagecreatefromgif and imagecreatefrompng. Instead, this is the way to go:

<?php
$src = "http://www.varuste.net/tiedostot/l_ylabanneri.jpg";
$image = imagecreatefromstring(file_get_contents($src));
?>

My site allows anonymous uploads to a web-accessible location (that will execute a script if it finds one).

Naturally, I need to verify that only harmless content is accepted. I am expecting only images, so I use:

<?php
  $im = $imagecreatefromstring($USERFILE);
  $valid = ($im != FALSE);
  imagedestroy($im);
  return $valid;
?>

[Editor's note: BMP will be supported as of PHP 7.2.0.]

In case it's not obvious from the lack of "imagecreatefrombmp()" in GD, this function cannot handle plain old BMP files either.

imagecreatefromstring does not appear to support WebP images (tested on PHP 7.2.10, with GD 2.1.0 and GD WebP support enabled)

So you guys don't spend an hour trying to figure out why your script keeps running out of memory when you're using this or the other imagecreatefrom functions.  GD uncompresses the image when you use these functions, and this can lead to your script running out of memory.

If you download a rawimage save it on your computer to jpeg so the file size comes down, GD will automatically convert it to the raw and you can possibly run out of memory.

Create an image resource from file, without knowing image type:

<?php
function imagecreatefromfile($imagepath=false) {
    if(!$imagepath || !$is_readable($imagepath) return false;
    return @imagecreatefromstring(file_get_contents($imagepath));
}
$img_resource=imagecreatefromfile($imagepath);
?>

I use dynamically generated images that require a little touch-up before being displayed. So essentially I do some base work, then store the images in a memory cache (APC), reload the images again from the cache later on "into" GD, do final processing and then display the image. 
Since I wanted to avoid a lot of disc access I used the output buffering functions:

<?php
   // Do your image processing stuff
   
   // Start buffering
   ob_start ( );
   ImageGD ( $hImage );
   $sImage = ob_get_contents ( );
   ob_end_clean ( );

   // Put stuff into cache
 
   // Reload from cache and recreate image
   $hImage = imagecreatefromstring ( $sImage );

   // Do final editing stuff and output image
?>

Of course this is a condensed example but I just wanted to share the idea.