If I understand what you want to do right, you can use the ArcIIS Server RESI API to do this. Using the REST API, you can request a single image from the ArcGIS Server map service and specify the bounding box, image size, image file type and many other parameters - and you will return one image of this size, type, etc.
The map service may be the one you published on your ArcGIS Server (9.3 or later). A service can be cached or dynamically. You can also use ArcGIS Online services, even to understand this. So, for example, go to the following URL:
http://server.arcgisonline.com/ArcGIS/rest/services/World_Street_Map/MapServer
In the "Supported Operations" section at the bottom of the page, go to "Export Map". Enter the size in the "Image Size" text box (for example, 1200, 1200) along with any other parameters and click "Export Image".
You can feel the acutal URL arguments by looking at the resulting URL for your request. Here you can get information about the parameters (for example, for the export operation): http://help.arcgis.com/en/arcgisserver/10.0/apis/rest/export.html
Now you mentioned the creation of a web service, so for this you can programmatically just click directly on the REST API. Or, you might want to create your own service as a wrapper around these REST API calls.
Hope this helps, and I understood your question correctly.
source share