Effect composer custom image handling is not very intuitive currently.
Suggested changes:

• All effects in all projects will have same custom images available for selection.
• Store preview images in some central location instead of current project. (documents or appdata)
• Allow arbitrary number of custom images to be defined
• Add button to custom images to allow user to remove them
• Custom image path is no longer stored into .qep, just the current selected image path. If the selected image doesn't exist, fall back to default images (like it works already).