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: http://i40.photobucket.com/albums/e218/kureshii/box%20images/faq/adv_desc/jpegwindow.png
- Adjust the quality as high as possible until it's just under <150kb.
Compression in Photoshop
- Go to File > Save for Web & Devices http://img502.imageshack.us/img502/8515/saveforwebservices.png
- Change the file type to drop down in the drop down box
- Adjust the quality to find the right file size (Look at the bottom left under the preview) http://img843.imageshack.us/img843/2888/dialogbox.png