@vit-hofman,
This is a very small image, the size of this image is 75x100px, and it becomes blurred on 100% width. So, you should turn off the 100% width on mobile devices to solve this issue. Here is the CSS code, put it in Dashboard > Forums > Settings >Styles > Custom CSS Code textarea, save it, delete all caches and use incognito/private tab of the mobile browser to check, otherwise you'll not see any changes:
@media screen and (max-width: 800px) {
#wpforo #wpforo-wrap figure[data-attachids] {
max-width: 100% !important;
width: auto;
margin-bottom:10px
}
#wpforo #wpforo-wrap .wpforo-post-content div.wpfa-item.wpfa-img-boxed img {
max-width: 100% !important;
width: auto !important;
}
#wpforo #wpforo-wrap .wpforo-post-content div.wpfa-item.wpfa-img-boxed {
max-width: 100% !important;
width: auto !important;
}
}