i've created a background image for my horizontal Xfce panels to give them nice metallic OSX-like look. the image itself is seamless, yet i've noticed some vertical glitches (like texture "seams") under certain panel widgets. this mainly concerns the window buttons widget. while using the flat button style, the background image seems to be rendered with invalid offset in various places (depending on which window button has been clicked). the notification area plugin and the clock - which i also use - seem to affect background rendering in a similar way. i have a separate panel with just launchers and separators and this one seems OK (no glitches at all). therefore it appears to be related to particular plugins. i can provide screenshots if necessary.
Created attachment 6017 Image to use for testing
Created attachment 6018 Example of a panel with the test image applied as a background
I've also encountered this problem. Instead of showing the expected part of the panel background underneath themselves, certain plugins (including, but not limited to, the Indicator and Weather Update plugins) actually show the start of the image. The attachments show a long image that can be used as a panel background and an image of a panel with that background applied and a random assortment of plugins. Some plugins (such as the Separator, Window Buttons and Directory List) continue the background image as expected, whereas others show the start of the image for the length of the plugin. This makes it very difficult to use any image that varies across its length or that differs in some way at the leftmost edge as a panel background, preventing a lot of possible visual effects.
Unfortunately this is not really (easily) fixable. You can only use pictures that are repeatable as external plugins (the ones you mentioned are all externals) do not respect the offset of a background picture (this is actually not that trivial to implement and considering the benefit, probably not worth it).
I honestly don't think we'll ever fix this, so no offense, but just to be clear I'm closing the issue instead of letting it rot.