Can you supply the code you've used to build the button and insert the image? Where is the image stored?
Make sure your image has the 'Build Action' set to 'Resource':
and declare the image in the config.daml as shown (make sure to replace the <your_DLL_Name> with the name of your add-in):
<button id="TaxParcelPlenary_DisasterFlood" caption="Flood Disaster" className="DisasterFlood" keytip="B3"
loadOnClick="true"
largeImage="pack://application:,,,/<your_DLL_Name>;component/Images/DisasterFlood32.png">
<tooltip heading="Flood Disaster">
Adjust the Tax Assessment after a declared Flood Disaster<disabledText />
</tooltip>
</button>