The main issue here is that the image handler just doesn't like BMP files. It handles standard web formats (jpeg, gif, and png), but I never added BMP support. The preview that shows the bmp is simply showing the raw file before processing it for a thumbnail.
If one enables high resolution screenshots in GW2, it defaults to BMP format. Now if I want to upload screenshots to my site, I have to open them in a photo editor and save as a different file type first.
The problem with bitmaps is their size. These other formats are used because they use compression to make themselves much smaller. Thus, your page gets loaded much faster. If you are using Windows, just load your Bitmap into "MS Paint" and then save it back as a JPG or PNG file. It is very simple.
