Got this answered on the BeerSmith site.....
The beer glass image for the different SRM colors in BeerSmith is:
Width: 297 pixels
Height: 512 pixels
Display aspect ratio: 0.580
Bit depth: 32 bits
Your browser will display gif or jpg as well.
The html code in the report resizes the image to height='128' width='75' so you should just need to worry about the aspect ratio of your image.