With the image object you are given 4 choices of how to access the image. The bad thing is that in order to change the image you need to change it on every report and it adds to the overall size of the RDL file, which depending on the image sizes could cause issues down the road. The good thing about this is that the report always has access to the image. OK it sounds like your company is using embedded images, again this is a bit of a guess, but if it gets uploaded with the report then that's probably what happens. It took me a while of playing with the different sizes and such to come up with this method, your mileage may vary, but basically just keep playing with the different sizes of the tables, header info until you find something that works. Then you can sort it or do whatever you need to as you would any other excel spreadsheet. That way when I export the report, the data in the body is the same size as the header so you don't get all of the merged cells. 25 in at the end of the header section with nothing in it. Then I make sure that the tables in my body are exactly the same width. What I've done in cases like this is that I create a report header with the information in textboxes etc that span the entire width of the report.
I take it your issue is less about the cells merging and more about issues with attempting to sort the report using the excel tools and receiving an error something to the effect of the merged cells needing to be the same size or some such?