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 a dialog box.
- Adjust the quality as high as possible until it's just under <150kb.
Compression in Photoshop
- Go to File>Save for Web & Devices
- 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)