Hi
1st create the report using the wizard that uses the default image type.
Go to the html source directory and find the image.
Copy and rename it to a jpg / bmp / png etc
Next create a custom template report.
Once you are at the selection criteria - select the Graphic File from the Available items on the left.
Import the graphic image as a .jpg / bmp /png etc
continue selecting reporting options.
once done - save as a new template
Go back to your model and select the newly created reporting template and continue as per normal.
Alternatively
I know this is not the most elegant solution but you could try the following.
1) Go the the directory where the html report files was saved.
2) copy and rename the image file to the desired format i.e .gif / .jpg
3) go to the html link in the report where the image is displayed.
4) right-click the image and select inspect element
5) check what the name of the source .html file is
6) edit the html page and rename the image in the .html file to what you changed it to.
7) save and refresh
hope this helps
regards
JC