I was reading this question ( How to check if an image file is clean? ) and forest's answer suggested converting the image to a "trivial pixelmap format, such as PPM". Such a format was considered "simple enough that it would be infeasible to exploit".
The question is: what makes a PPM file safer? Is it because there's no room to hide a payload, without becoming too noticeable at first sight? Or is it because the format is so simple to parse, that we can suppose our software will be less likely to be vulnerable, when opening such files? And most importantly, is it even true that we can consider PPM files (and maybe also BMP) to be safer than, say, more complex formats like JPEG or PNG?