You can use the <#imgsize> tag in the name of the image to keep the aspect ratio.
Please take a look at the "Images" demo (in report section) for an example, it show not only how to keep aspect ratio, but other possibilities, like keeping the width, keeping the height, resizing to 50%, centering the image, etc)ç
Also note that in the demo we use expressions, so the tags are in the Config sheet, ans instead of naming the image directly <#Employees.Photo><#ImgSize> we name it PhotoAutoSize and then define the expression PhotoAutoSize to be the full name. This is not such a bad idea, espceially because editing tags in the image name is kind of difficult.
Regards,
Adrian.