We've just added an !important statement in the code, then deleted the caches and it started to work fine. 

Brilliant its OK.

Can I ask what you did so I can learn from what I was missing

Please delete the browser cache and check again. The issue should be solved. 

details sent as requested, thanks, Much appreciated.

p.s. I've put everything back as it was before. 

Please send the admin login details to infogvectors.com email address. We'll try to help you to solve the issue. 

Hi, I cleared fast velocity cache, then removed the plugin entirely. Cleared forum cache and ctrl f5 twice. Still got the problem. I really don;t know what else to do.

Please see the screenshot below:

This is a cache issue for sure. The CSS code shown on the screenshot haven't been overwritten by the code provided in the topic. 

Please find a way to reset all modification/cache files and check again.

To clarify

1. The forum attachments were working fine after you supplied the CSS.
2. Yesterday I reported the flashing avatar
3. Yesterday evening you said apply the 1.7.2 release 
4. I applied 1.7.2 and did not remove the CSS
5. The flashing avatar problem remained, BUT now I had the attachment thumbnail problem back
6. I tried removing the CSS to see if it was causing the problem, it made no difference
7. I put the CSS code back

So I'm right back where I started 2-days ago.

Hope that helps explain

Don't follow you well. Let's step by step understand the issue.

1. Is the following issue solved after you updated the plugin?

This problem only impacts mobile phone browser viewing my forum. When going back a page, you get a flash of a user's avatar full-frame!!

2. The update doesn't include the CSS code below. You should not remove it. Just keep it in the "Custom CSS code" textarea.

@media screen and (max-width: 800px){
   #wpforo-wrap figure {
       width: auto;
   }
}

Anticipated you saying that and and it still fails as described.