Please correct me if I'm wrong here, but I believe "impact" is an "all caps" font.
Although I do think it's a classy-looking font--
I WOULDN'T USE IMPACT FOR A HEADLINE FOR THE SIMPLE FACT THAT ALL CAPS=SCREAMING!
As far as size goes...
I personally choose a size that fits well with the page width.
If it's a narrow page, I would go smaller.
If it's wide, I prefer a larger font size.
I've never given much thought about folks who have their images turned off.
Very good question!