created an attachment (id=168362)
screenshot

description:
chart background image only can be previewed once in chart builder

build:
2.6.0.v20100513-0630

steps to reproduce:
1. new a chart
2. format chart->chart area->background
3. embedded a image and preview
4. reopen background image and check the preview

actual result:
preview button is disabled

expect result:
support embedded image preview commonly
by opening the "select image" dialog, the current image will be automatically displayed in the preview area, which can be updated by each click on the "preview" button.
image always automatically previewed. verify the bug.