Originally Posted by unmarkedhelicopter
I believe that whatever picture you add to an Excel File (or Word Doc for that matter) it is stored as the same internal format (unfortunately it's a .bmp i.e. uncompressed).
I'm not convinced about that because of specific experiences I've had. When I started my last job there was a manual that several people had created using screenshots. They would do the typical, "hit the PrtScn button" and then paste into the document. Now, THAT is definitely using the largest size possible. Then, if I created png files of those same screenshots before inserting, the size would be many, many Kilobytes less.
You can run a quick test, too. Take a bmp file and convert it to png. Then, open a new .doc file and insert one of the pics into it and save it. Then, create another new .doc file and insert the other one and save it. Check the file sizes. I just did the same test with a 6"x3" screen capture saved from SnagIt and saved the same pic as two different types and then inserted each into a separate document. Just with one pic there was 4Kb difference.
The interesting thing is that the Word documents that I inserted the pics in were:
53 KB - with BMP
47 KB - with PNG
And the original sizes of the image files before inserting were:
BMP - 947 KB
PNG - 30 KB
So Word is doing something to it as well when you use the Insert > Picture > From File to compress it as well.