The acp_icons.php script incorrectly handles SVG smilies or topic icons that use viewBox without explicit width/height attributes. Most SVG images are in this format to insure the image will automatically fill the available area.

Right now, such SVG images will be set by phpBB to be 32x32 regardless of their aspect ratio. In addition, such images will be displayed incorrectly in the ACP and on the frontend. Attached is a ZIP containing a sample SVG file that can be added to the images/smilies or images/icons directory that demonstrates the issue in either the ACP icons or smilies area, and also when the image is displayed on the frontend.

The ZIP also contains a DIFF that fixes the issue.