There has to be some level of fair use or copyright involved, no? You are taking corporate images, modifying them, and then distributing their modified brand/logo potentially without their consent. If they don't like the results, they could sue you.
'Fair use' only applies to the copyright implications of distributing these images; the far murkier bit of IP law that this gets into is trademark licensing. That's far more likely to be the issue here.
Not really modifying it. But they may have branding and licensing guidelines that a service like this might not meet which would likely result in a cease and desist rather than an immediate lawsuit.
Or at least, I feel that would be the case.