if there are no changes within your resouces (you can put bitmap and font resources into separate units), then there is no need to flash the address range again. But I'm not sure if you can exclude a certain address range from download within IAR workbench.
Anyhow, this can be a dangerous approach - please consider that also the resources of your project may change and thus the entire binary will change!
In order to reduce the flashing timg in case you are using fonts with thousands of characters, you can try to reduce the Ranges of your fonts to a minimum - just for development and add the whole character set once the project is more or less final.
Furthermore, you should try to develop the GUI appplication within Embedded Wizard Studio by using the Prototyper so that downloading to the target is only necessary from time to time...