[buttsie]

I recently finished trialing kasperskys security suite and had no issues uploading to the usual suspects in imagevenue or imagebam

Depending on which host it can also easily be a browser issue

Internet explorer is easily the worst for throwing up compatibility issues
with all manner of sites

Even more reliable browsers like Firefox can have issues that relate to the add-ons
Disabling them all via help - start with add-ons disabled is a good first option

Ad-block plus has been known to cause issues on occasions with sites

People forget when their running No script they have to allow the site access
otherwise it wont load entirely resulting in greyed out areas or
on filehosts captcha codes not working even after being input