Finally it turned out. In case someone has a similar request; You can use hyperlinkTooltipExpressionto set alternative text. Something like that;<image isLazy="true">
<reportElement positionType="Float" x="0" y="0" width="100" height="100"/>
<imageExpression class="java.lang.String"><![CDATA[$P{imageToBeShown}]]></imageExpression>
<hyperlinkTooltipExpression><![CDATA["Description goes here"]]></hyperlinkTooltipExpression>
</image>
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