Copy an image you choose in "custom" folder and paste in previous folder, then remove old image file and rename the new one you selected to the one you deleted.

Example : Remove saturn in night sky
1. Copy "sky5_night_no_saturn" image in custom folder and paste in environment folder
2. Delete old image that named "sky5_night"
3. Rename the one you selected from "sky5_night_no_saturn" to "sky5_night"
4. Reload resource pack (F3+T)