Don't pay attention to my previous post, I was completely wrong :-)
The [Tool] blocks themselves are read either from the siteaccess toolbar.ini.append or from the default toolbar.ini (see kernel/visual/toolbar.php line 56-63 eZ publish 3.8.3), but the [Tool_description] blocks are only read from the default toolbar.ini (line 250-253).
Hi
Pascal, Im not sure what U want to get, but I have a guess, and try change name of your image file like: mytool_line.png (instead of mytool.png) or modify visual/toolbar.tpl, because there is:
{section show=eq($toolbar_position,right)} {* <-- only for right toolbar without '_line' *}
<img src={concat( "toolbar/", $Tool.name|wash, ".png" )|ezimage} alt="{$Tool.name|wash}" />
{section-else}
<img src={concat( "toolbar/", $Tool.name|wash, "_line.png" )|ezimage} alt="{$Tool.name|wash}" />
{/section}
