Christopher Brown answers WRONG.
The alt tag is supported using the ToolTip property.
I use the alt tag to set the height of the logo in JavaScript, depending on the image name "Logo xy", since it displays correctly in HTML QuirksMode, which is generated by SSRS.


Title Alt-Tag, JSON JavaScript ReportViewer.aspx.
- :
var arrimgLogo = document.querySelectorAll("img[onload^='this.fitproportional=true']");
if(arrimgLogo != null && arrimgLogo.length > 0)
{
var img = arrimgLogo[0];
// img.removeAttribute('height');
// img.style.maxWidth = '100%';
// img.style.maxHeight = '100%';
if(img.alt != null && img.alt.toLowerCase() == "Logo SwissRe Left".toLowerCase())
{
img.height = 35;
}
else
{
var tP = img.parentElement;
//console.log(tP);
if(tP != null && tP.tagName == 'DIV' && tP.style != null)
{
tP.style.textAlign = "right";
// if (parseFloat(tP.style.minWidth) != 0) tP.style.width = tP.style.minWidth;
// if (parseFloat(tP.style.minHeight) != 0) tP.style.height = tP.style.minHeight;
} // End if(tP != null && tP.tagName == 'DIV' && tP.style != null)
}
} // End if(arrimgLogo != null && arrimgLogo.length > 0)