custom symbol c#: mislay float background?

i'm perplexing tradition symbol form (which formborderstyle = none) controlling visible studio 2005. i have 3 states symbol images an imagelist related button.

this.btnclose.autosize = false;
this.btnclose.backcolor = system.drawing.color.transparent;
this.btnclose.backgroundimagelayout = system.windows.forms.imagelayout.center;
this.btnclose.flatappearance.bordersize = 0;
this.btnclose.flatstyle = system.windows.forms.flatstyle.flat;
this.btnclose.forecolor = system.drawing.color.transparent;
this.btnclose.imagekey = "disabled";
this.btnclose.imagelist = this.imagelist1;
this.btnclose.location = new system.drawing.point(368, -5);
this.btnclose.margin = new system.windows.forms.padding(0);
this.btnclose.name = "btnclose";
this.btnclose.size = new system.drawing.size(31, 31);
this.btnclose.tabindex = 0;
this.btnclose.usevisualstylebackcolor = false;
this.btnclose.mouseleave += new system.eventhandler(this.btnclose_mouseleave);
this.btnclose.click += new system.eventhandler(this.btnclose_click);
this.btnclose.mousedown += new system.windows.forms.mouseeventhandler(this.btnclose_mousedown);
this.btnclose.mousehover += new system.eventhandler(this.btnclose_mousehover);

private vacant btnclose_mousehover(object sender, eventargs e)
{
    btnclose.imagekey = "enabled";
}

private vacant btnclose_mousedown(object sender, mouseeventargs e)
{
    btnclose.imagekey = "down";
}

private vacant btnclose_mouseleave(object sender, eventargs e)
{
    btnclose.imagekey = "disabled";
}

all working, nonetheless there's catch. whenever i pierce rodent float symbol i unequivocally irritating grey background.

how i mislay that?