One possible reason for this issue is that bmp image has bigger size than jpg images, and in general the maximum upload size set in the webconfig is 4mb. My suggestion is to increase this size and check, if this fixes your problem. The setting for this size is the following:
the Telerik team
Registration for Q1 2011 What’s New Webinar Week is now open. Mark your calendar for the week starting March 21st and book your seat
for a walk through all the exciting stuff we ship with the new release!