In the ACP, you can define settings for signature:

• Enable / disable the signature feature itself
• allow BBcode, [IMG], [FLASH], smilies or links in signatures
• Set maximum values: length, number of links, number of smilies, font size, height and width for images and "flash" objects.

The last two settings are problematic: phpBB simply ignores the defined maximum height and width for signatures images. Even if I define 300px for width and 60px for height (for example), I can add an 8K image (example; https://axeetech.com/wp-content/uploads/2014/10/alien_orbit7680x4320.jpg ) in my signature

phpBB 3.3.9 and 3.3.10 are equally affected.

I don't know when the bug appeared, but I suspect the feature was actually removed for signatures when the image dimensions check has been removed for posts (see Post settings: maximum height and width now applies only to flash/video objects, not images).