radu,
For more information about the features of the ILA mod, you should talk to the author, Spuds. He is a good guy and a former SMF team member.
As for portal resizing, SimplePortal does this in a very special way, and I know Spuds was not thinking of SimplePortal when he wrote it. And,, of course, [SiNaN] was not thinking of Spuds' mod when he wrote SimplePortal.
But that does not mean you cannot bridge the gap.
SimplePortal uses some javascript to find all images that contain (if you are using SMF 2.X) "bbc_img sp_article" in the list of classes. How does it get this way? In PortalArticles.php, it looks for 'class="bbc_img"' and replaces it with 'class="bbc_img sp_articles"'.
To figure out what to do next, please have a look at the HTML code produced by your custom tag.
If it produces 'class="bbc_img"' , you are set. But I am guessing that if it did, you would not be asking this question. Perhaps it produces something slightly different, like 'class="bbc_img spud_special_class"'. Please take a look at what the current version of PortalArticles.php is looking for and what your tag is producing to find a solution.
Somehow, I feel I have seen this question asked before. And I am pretty sure the answer is pretty easy. Depending on what we find, you may want to make a small change to PortalArticles.php or else to parse_bbc in subs.php.
Please have a look and report back here what you find.