Hello I tried and on my linux system, its possible to gimp-edit even inline images.
Maybe there is an issue in scribus or on your system, but sometime it's easier to ask some experienced person IRL. For me, it was a necessary step before being able to use gimp because i could not grasp even its most basic use for a long time. So maybe some IRL help would help you using scribus. However, i dont think you really did the export of the 'inline' version to a 'linked' file version. Doing so requires to click on the allready clicked 'inline' option of the 'Image' submenu in the context menu. Have you done so ? The proposed name for the created file has not a "draft" prefix as you describe it, but when i did it, the name was something like 'scribus_temp_pdf_H16308.png' And i dont get what is it you call 'online'. JLuc
