config.xml file contains information about layers configuration something like this: <basemaps>
<layer label="Streets" type="tiled" visible="true" alpha="1"
url="http://server.arcgisonline.com/ArcGIS/rest/services/World_Street_Map/MapServer"/>
<layer label="Aerial" type="tiled" visible="false" alpha="1"
url="http://server.arcgisonline.com/ArcGIS/rest/services/World_Imagery/MapServer"/>
<layer label="Topo" type="tiled" visible="false" alpha="1"
url="http://server.arcgisonline.com/ArcGIS/rest/services/World_Topo_Map/MapServer"/>
</basemaps>
<operationallayers>
<layer label="Boundaries and Places" type="tiled" visible="false"
url="http://server.arcgisonline.com/ArcGIS/rest/services/Reference/World_Boundaries_and_Places_Alternate/MapServer"/>
<layer label="Fires" type="feature" visible="false" alpha="1"
info="widgets/InfoTemplates/SimpleInfoWinWidget.swf"
infoconfig="widgets/InfoTemplates/IWT_Fires.xml"
url="http://sampleserver3.arcgisonline.com/ArcGIS/rest/services/Fire/Sheep/FeatureServer/0"/>
</operationallayers> add imageFormat property to your layer: <basemaps>
<layer label="Streets" type="tiled" visible="true" alpha="1"
url="http://server.arcgisonline.com/ArcGIS/rest/services/World_Street_Map/MapServer"/>
<layer label="Aerial" type="tiled" visible="false" alpha="1"
url="http://server.arcgisonline.com/ArcGIS/rest/services/World_Imagery/MapServer"/>
<layer label="Topo" type="tiled" visible="false" alpha="1"
url="http://server.arcgisonline.com/ArcGIS/rest/services/World_Topo_Map/MapServer"/>
</basemaps>
<operationallayers>
<layer label="Boundaries and Places" type="tiled" visible="false"
url="http://server.arcgisonline.com/ArcGIS/rest/services/Reference/World_Boundaries_and_Places_Alternate/MapServer"/>
<layer label="Fires" type="feature" visible="false" alpha="1"
info="widgets/InfoTemplates/SimpleInfoWinWidget.swf"
infoconfig="widgets/InfoTemplates/IWT_Fires.xml"
url="http://sampleserver3.arcgisonline.com/ArcGIS/rest/services/Fire/Sheep/FeatureServer/0"/>
<layer label="MyLayer"
type="dynamic"
visible="true"
imageFormat="png32"
url="my layer url"/>
</operationallayers> Find MapManager.mxml in FlexViewer. This file contains layes adding business logic. As you see, only such types of layers support imageFormat proprty: "dynamic", "image", "arcims", "wms". You can change/add this property in this file. More information in Flex API reference. If you are editing the config.xml file for Flex Viewer 2.4 note this correction to the above code reference: Use lowercase like this; imageformat="png32"
... View more