[kmike]

Excellent hack.
I see a couple of small problems with it though:

1. it doesn't handle additional image types supported by getimagesize() PHP function, e.g. .SWF, .PSD, and .BMP. First two are probably non-issue, but our forum has .bmp attachments enabled, and this hack will generate an empty resized image for them
2. a huge image can cause "out of memory" PHP error on ImageCopyResampled() call, and the hack doesn't handle these situations (I think 500 errors cited in this thread are caused exactly by this - check your error logs for PHP errors)
3. the attachment file size stored in the db is wrong - the stored number is the size of original attachment, not the resized one. the total space occupied by member's attachments will be wrong, which may cause problems if attachment quotas are defined for usergroups
4. some memory could be freed during script execution by adding ImageDestroy() calls in the proper places
5. I think UnsharpMask() call is only needed if the attachthumbs option in admin CP set to "Yes - sharpen" (to be consistent with thumbnails handling)
6. there's no need to mess with "require_once('./includes/functions_image.php');" line at all. The file is only included once, if needed

I've fixed these problems, and also did some code cleanup.
You can uncomment error_log() lines and change email to your actual address to get some idea how this hack performs.

				// HACK: auto resize uploaded images
				switch ($imginfo[2])
				{
				case 1:
                			$im = ImageCreateFromGIF($attachment);
					break;
				case 2:
                			$im = ImageCreateFromJPEG($attachment);
					break;
				case 3:
                			$im = ImageCreateFromPNG($attachment);
					break;
				// no way to handle these image types
				default:
					// error_log("Couldn't resize $attachment ($filesize bytes)", 1, "email@yourdomain.net");
					@unlink($attachment);
					eval('$error = "' . fetch_phrase('attachbaddimensions', PHRASETYPEID_ERROR) . '";');
					$errors[] = array(
						'filename' => $attachment_name,
						'error' => $error
					);
					return false;
				}

				$w = $imginfo[0];
				$h = $imginfo[1];
				$width_factor = $w / $maxattachwidth;
				$height_factor = $h / $maxattachheight;
				if ($width_factor > $height_factor)
				{
					$nw = round($w / $wdth_factor);
					$nh = round($h / $wdth_factor);
				}
				else
				{
					$nw = round($w / $height_factor);
					$nh = round($h / $height_factor);
				}

				$ni = @ImageCreateTrueColor($nw, $nh);
				if ($ni OR !@ImageCopyResampled($ni, $im, 0, 0, 0, 0, $nw, $nh, $w, $h))
				{
					// failed to resize this image
					// error_log("Couldn't resize $attachment ${w}x${h} to ${nw}x${nh} ($filesize bytes)", 1, "email@yourdomain.net");
					@unlink($attachment);
					eval('$error = "' . fetch_phrase('attachbaddimensions', PHRASETYPEID_ERROR) . '";');
					$errors[] = array(
						'filename' => $attachment_name,
						'error' => $error
					);
					return false;
				}
				@ImageDestroy($im);
				if (PHP_VERSION != '4.3.2' AND $vboptions['attachthumbs'] == 2)
				{
					require_once('./includes/functions_image.php');
					UnsharpMask($ni);
				}
				switch ($imginfo[2])
				{
				case 1:
					@ImageGIF($ni, $attachment);
                			break;
				case 2:
					@ImageJPEG($ni, $attachment, 70);
					break;
				case 3:
					@ImagePNG($ni, $attachment);
					break;
				}
				@ImageDestroy($ni);
				$filesize1 = @filesize($attachment);
				// error_log("Resized $attachment ${w}x${h} to ${nw}x${nh} ($filesize to ${filesize1})", 1, "email@yourdomain.net");
				$filesize = $filesize1;
				// /HACK