Is it possible to add a simple image or graphic to the map canvas in QGIS (e.g. a symbol marker)?
You can add a symbol to the map rather than the print output in Composer. QGIS 1.8.0 Lisboa has some generic symbols included. If you go to Settings/Style Manager then ADD then Symbol Layer Type and choose SVG Marker you can choose from a number of symbols there and save the one you want.
Once you have your chosen symbol you need to add a Shapefile Layer in Layer/New and choose New Shapefile Layer. Give the layer the attributes you require, ensure it is POINT type rather than Polygon, and save. The new layer will appear in the Layers area. To add your new symbol ensure you are working in the new layer. Toggle editing on and choose the Add Feature tool. Click in the position you want your new symbol and you will get a small round generic point symbol.
To get your chosen symbol you need to right click the layer, choose Properties and then the Style Tab. The symbol you chose earlier should be there ready for you to choose.
Assuming that you are using at least QGIS Wroclaw (1.7.4), images or graphic in can be added in composer:
File -> New Print Composer -> Layout
and you will find all the options you are looking for.