You mean an exploit payload embedded in an image, and pwning a system parsing that image through python PIL? While there’s never a 100% chance of anything, you’re more likely to be struck by lightning than this coming to pass and at that point you’re at more security risk at using the internet altogether.
Image processing libraries are used at the forefront of almost all web services, including lemmy and are extraordinarily robust. I really don’t have the time to go at this in depth, but if you are familiar with this stuff you will know how extraordinary such an exploit would be and its existence would be causing massive chaos all over the world.