Just curious, but for what purpose are you looking to post the pic up for? You can reduce the size of the picture a couple of ways, depending on the file type and actual size of the image itself. Have you tried opening the image in your paint software and resizing it that way? Bitmap(.bmp) files are generally large and are altogether not allowed as attachments here.

Just make sure while you're fiddling with it you save the file under a different name so you can retain the original. If you're still having problems, post back and let me know. A link to the item in the museum would help on my end a bit if you need further assistance.