I do not think it is necessary to touch your miniBB's index.php at all. Depending on your site's set up, I have 2 suggestions (pick one or the other, not both!)
** Suggestion A - if your site's home page is currently NOT your miniBB home page:
1) install miniBB's 1stpagenews addon script
2) configure the addon script
3) customize the addon script's templates, if necessary
4) call/include the script from your site's home page, not your miniBB home page. The latest forum topics will display on your site's home page.
** Suggestion B - if your site's home page IS currently your miniBB forum home page:
1) create a new index page (name it index.html or php or whatever - depending on your preference) - this page will become your site's new home page. Design the page so that it has a 3-column layout similar to the page you directed us to in your original post. Leave the centre column free for displaying latest posts.
2) rename your miniBB forum's home page (currently named index.php) to forum.php or anything (just make sure it differs from your site's home page that you created in step 1)
3) in miniBB's setup_options, update $indexphp (check miniBB Manual for instructions) to point to the renamed file (see step 2).
4) install 1stpagenews addon script. call the latest discussions on your home page.
5) configure the addon script
6) customize the addon script's templates, if necessary
7) call/include the script from your site's home page (the index file created in step 1, NOT the file renamed in step 2).
The latest forum topics will display on your site's home page.
As to getting images to appear in the summary for each recent topic, you will need to ask Team for advice.
All the best -