BakaBT only allows images for descriptions that are <150kb in size. If your image is over the limit, you'll have to compress it to a file size that's acceptable.
JPEG is a format for lossy compression of images. This is the simplest and most wide-ranged used image format for compression.
Compression in GIMP
In GIMP, choose to save as a JPEG. You should see this dialog box: File:Http://i40.photobucket.com/albums/e218/kureshii/box images/faq/adv desc/jpegwindow.png