When it comes to the topic of the 'awrah of men scholars hold different positions. Even if the majority seems to agree about one.
In the sum there are four or five different views (or different opinions including all details):
The 'awrah of men is only the privat parts (buttocks and the private parts in the front) which means that the tighs are not part of ...
Is it tohmat to assume that two people are together if you’ve seen pictures of them together?
No this would not be tohmat. Because as @Medi1Saif said, tohmat means accusation. If you are just assuming in your mind, it is not an accusation which has to be verbal.
What you are describing is non-verbal "Zann" (suspicion):
"Beware of Zann (suspicion), for ...