How can I use my own raster files?

Set up a WMS server and use the WMS layer model. Any WMS server that supports the raster types you want to process can be used. See http://www.geomajas.org/application-configuration for an example of a wms layer factory. After configuring the factory you can add the layer xml, for example :

<layer xmlns="http://geomajas.org/schemas/configuration/"
    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xmlns:xsd="http://www.w3.org/2001/XMLSchema" id="wms"
    xsi:type="rasterLayerInfo">
    <label>WMS</label>
    <visible>true</visible>
    <layerType>1</layerType>
    <crs>EPSG:31370</crs>
    <viewScale>
        <min>0</min>
        <max>50</max>
    </viewScale>
    <maxExtent>
        <minX>21900</minX>
        <maxX>258950</maxX>
        <minY>153100</minY>
        <maxY>244130</maxY>
    </maxExtent>
    <maxTileLevel>16</maxTileLevel>
    <rasterLayerFactoryRef>someWMS</rasterLayerFactoryRef>
    <layerName>someLayer</layerName>
    <tileWidth>256</tileWidth>
    <tileHeight>256</tileHeight>
    <style>1</style>
</layer>