1) For the images in posts, find this in index.css:
/* Auto resizing of images in posts, pm's, etc. */
/* Change max-heights to suit your preference. */
.bbc_img {
height: auto;
width: auto;
max-width: 100%;
max-height: 640px;
}
.signature .bbc_img {
max-height: 150px;
}
What is probably screwing you up is the max-width and max-height. The former won't allow an image wider than the post content area. The latter obviously restricts height to less than 641px.
If you remove the max-width and max-height you should be able to set any size on the image's BBC tag. You'll then need to allow large images to overflow the post content area, so find this:
.inner, .ignored_prompt {
overflow: hidden;
margin-top: 4px;
padding-top: 9px;
border-top: 1px solid #99a;
}
And change the overflow from hidden to auto. That should fix it.
2) For the recent posts columns, find this:
.recent_posts {
display: flex;
flex-wrap: wrap;
margin: -5px 2px 0;
}
.recent_posts p {
flex: 1 1 auto;
box-sizing: border-box;
width: 49%;
margin: 0 .5%;
padding: 6px 0;
overflow: hidden;
font-size: 1.2rem;
text-overflow: ellipsis;
border-bottom: 1px solid #ddd;
}
Change it to this:
.recent_posts {
margin: -5px 2px 0;
}
.recent_posts p {
box-sizing: border-box;
margin: 0 .5%;
padding: 6px 0;
overflow: hidden;
font-size: 1.2rem;
text-overflow: ellipsis;
border-bottom: 1px solid #ddd;
}