This is not correct.
As far as standard attachments in phpBB go, each attached image is inside a
<div>
tag which is the reason why attached images are one per line. But IMG tag images can be arranged side by side normally.
As far as the extension
Inline images in attachment goes when used with
Lightbox, the problem is with Inline images in attachment. The Inline images in attachment extension shrinks attached images to a size smaller than they originally were, which means they end up smaller than the threshold the Lightbox extension recognizes as needing to be resized.
For example, if you set up attachments to be a width of 400px, then you would also need to set up Lightbox to resize images >= 400px. But if you install the Inline images in attachment extension, your attachments will be scaled down to something like 350px, and thus Lightbox will ignore it.
So the two extensions are incompatible, and the solution would be for the author of Inline images in attachment to find a way to make attached images appear inline without also resizing them to smaller than original.
A temporary hack could be to just make your Lightbox resize width really small too, so that it just applies to almost everything.