You'll need to edit your tamplates/default/gallery_island.tpl template so that it custom displays the way you want it to display.
Right now it is displaying each image in its own table ROW
{section name=item loop=$items}
<tr>
<td class="{$style_side}alt-1" align="center">
<a href="{ubb url="ubb=showgallery&Number=`$items[item].POST_ID`"}"><img alt="{$items[item].TOPIC_SUBJECT}" title="{$items[item].TOPIC_SUBJECT}" {$items[item].WIDTH_HEIGHT} src="{$config.BASE_URL}/gallery/{$items[item].TOPIC_THUMBNAIL}" /></a>
</td>
</tr>
{/section}
Maybe update it to something like:
<tr>
<td class="{$style_side}alt-1" align="center">
{section name=item loop=$items}
<a href="{ubb url="ubb=showgallery&Number=`$items[item].POST_ID`"}"><img alt="{$items[item].TOPIC_SUBJECT}" title="{$items[item].TOPIC_SUBJECT}" {$items[item].WIDTH_HEIGHT} src="{$config.BASE_URL}/gallery/{$items[item].TOPIC_THUMBNAIL}" /></a>
{/section}
</td>
</tr>
This will make the change for all Gallery Islands.
You could add some margin to your image elements. Put a custom border around each image -- or make them round by using "border-radius: 50%;" in your image style.
Customize the code so that it works the way you want it to.