tldr: Decrease the limit number of posts to allow users to create attachments. And another suggestion
I cannot find permissions for PunBB member levels on here or PunBB forum/documentation, so some of the following is assumptions.
You cannot post attachments unless you have made 50 posts - a 'senior member'.
So as I understand it; the persons requiring the most help are new users, as they may not yet know the correct terminology of the problem the have, English is not their first language, etc. So screenshots would help them 'a picture is a 1000 words.'. Also by the time they reach 50 posts, they probably have sufficient knowledge not to have to post accompanying screenshots, the screenshots they now post are to help someone else. Yes they can link to external sites, but that's too many steps and complicates the process. It also doesn't encourage good behaviour either - a user can easily write more posts to increase their membership level by writing valid but wasteful posts - e.g. 'Yes I agree' or 'Thank you, this helped me too' etc.
By the time a member posts say 5 posts you can tell if they are a spammer, troll etc, so can a new level of membership be created, say 'Trusted Member'. You can still ban them anyway.
If no more levels can be created, change the 'senior member' level and rename it? It doesn't mean much anyway?
Also whilst researching this I noticed an unofficial extension on the PunBB site that notifies users if they have received a PM which has also been asked for on this site.
Thank you for your time.