Is designing internet page templates for displaying images, which may include images of mixed men and women, considered assisting in a forbidden act, knowing that the designer does not add the images himself, but rather the website owners do?

Scholars have differed regarding the ruling on photographing animate beings, and the more accurate approach involves elaboration: If the photography includes drawing 'awrah (private parts) or anything unlawful, it is unanimously forbidden. If it is free from such elements and there is a need for it, then it is permissible. If there is no need for it and it does not contain anything forbidden, then it is a matter of hesitation, and it is more pious to avoid it. Based on this, the ruling on your work as a web designer depends largely on the presumed content of the images. If it is highly probable that there is forbidden content, then your work is forbidden. Otherwise, it is permissible. If there is no strong probability either way, then it is a matter of hesitation, and it is more pious to avoid it.