Geoserver style with condition

How can I use a different image (OnlineResource) depending on the attribute value? I have a feature with a 'number' attribute, I need to do something like: if number = 0 then OnlineResource = image 1 else OnlineResource = image 2

You can use two rules with filters in your SLD:

  <FeatureTypeStyle>
     <Rule>
       <Name>local-road</Name>
       <ogc:Filter>
         <ogc:PropertyIsEqualTo>
           <ogc:PropertyName>type</ogc:PropertyName>
           <ogc:Literal>local-road</ogc:Literal>
         </ogc:PropertyIsEqualTo>
       </ogc:Filter>
       <LineSymbolizer>
         <Stroke>
           <CssParameter name="stroke">#009933</CssParameter>
           <CssParameter name="stroke-width">2</CssParameter>
         </Stroke>
       </LineSymbolizer>
     </Rule>
     <Rule>
       <Name>secondary</Name>
       <ogc:Filter>
         <ogc:PropertyIsEqualTo>
           <ogc:PropertyName>type</ogc:PropertyName>
           <ogc:Literal>secondary</ogc:Literal>
         </ogc:PropertyIsEqualTo>
       </ogc:Filter>
       <LineSymbolizer>
         <Stroke>
           <CssParameter name="stroke">#0055CC</CssParameter>
           <CssParameter name="stroke-width">3</CssParameter>
         </Stroke>
       </LineSymbolizer>
     </Rule>
   </FeatureTypeStyle>