Give this a try using built in icons from Office (get rid of buttonGroup
)
<customUI xmlns="http://schemas.microsoft.com/office/2006/01/customui">
<ribbon>
<tabs>
<tab id="toolRibbon" label="HMA-FUNCTIONS" insertBeforeMso="TabHome">
<group id="groupDocument" label="HMA-xml outputs">
<button id="aa" label="CORE" imageMso="MacroArguments" onAction = "HMA_CORE" size="large" />
<button id="ab" label="PLANT" imageMso="PictureBrightnessGallery" onAction = "HMA_PLANT" size="large" />
</group>
</tab>
</tabs>
</ribbon>
</customUI>
Reference: Office 2007 Icons Gallery