Thank you for your post. I apologize for the late reply.
In Layout Mode, select the Container field, pull down the Format menu and select "Graphic...". Be sure to check "Maintain original proportions". I usually set the Alignment to Center and Center, but I don't think this should affect how it displays on the web.
Can you tell me more about the images themselves? Are they JPEG? TIFF? What Browser are you using? Any information you can provide to help me replicate the problem would be appreciated.