When you place a sign with Quark installed, it opens a GUI that makes typing on the sign easier. This completely breaks the mod. I can successfully paste an image and click the place button, and can even see the preview. When I place a sign, however, it functions identically to if only Quark was installed. A boring, blank sign is placed with no fancy text and no image.

This can be worked around by either uninstalling Quark, or by disabling that specific feature in Quark's config file. I believe the issue is caused because Sign Picture is waiting for an event triggered when the vanilla sign GUI is opened, which Quark intercepts.

Go into quark's options in the Escape menu or main menu, under the client section disable improved sign edit. Hope this helps :)

That's exactly what I did, and it worked. I was hoping you could solve the conflict so I could enabled both features.